So click New. A template box opens up. Put in your dimensions. If you are just doing one headshot picture go ahead and make it 512x512 but for the headshot and long shot presentation i enter 700x700 and re-size it when i'm done.
Double click the (usually) white box in the tool box- (brown on mine cos i changed it to that) to select your chosen colour
Ok, so click on Edit and Fill with background colour. I usually make it brownish or grey but whatever you like. Some people use the ultra green background but i hate it, because i don't have time to edit tiny bits of green out of a sims hair etc! You can just use white if you are pasting onto a white template.
So click File>Open and go to your Myfolder/ElectronicArts/Sims3/Screenshot folder and select the sim you wish to edit, which will open in a new image. You can minimise the other one if you wish but do not close it as obviously we will be pasting our sims on this background.
So on your sim picture you just opened we want to erase the background behind it, first crop around the sim to make it easier by selecting the first tool in the tool box (the square) and start in the corner and drag across the sim, So in the tool box select the wand tool (top second last tool with yellow tip (see below).
With your mouse select layer>Transparency>Add Alpha Channel. (see above with the red dot guides) Then with the wand, click anywhere in the background of your sim, (NOT ON YOUR SIM) you should see dotted lines around the edge surrounding your sim, then click Edit and Clear, then there should be a checkered grey background behind your sim (see pic below). You will still see the dotted line around your sim, to make this go away (and you must do this) click anywhere outside the grey square. If there are pieces of background behind the hair (in mine there isn't) use the Select by colour tool top last in the tool box (a finger icon pointing atblue/red/green cubes) select the background in the hair, becareful not to select it on the sim just on the background area you need to get rid of. this selects everywhere that has the background colour. Use the eraser tool (the pink square( 5th down last across) and erase the background from your sims hair or wherever else there are still bits of background remaining. when you are happy with that again, you will still see the dotted line around your sim, to make this go away (and you must do this) click anywhere outside the grey square. You have now prepared your sim for pasting. See pic.
Now click Edit >(a box drops down), select Copy and go back to the first template we made with the brown background, and click Edit and paste. Your pasted sim should be there. Do not merge the layer yet , grab the move tool (blue cross)
Now after i drag my sim over and i am happy with the position again i click anywhere outside the box and the lines disappear and my sim is attached to the background.
Now leave this for the time being and again in Gimp click New> and go to your screenshots folder and select the long view sim you want to paste beside the main sim above. Go through the same steps as above the whole way down. Thats all it is. Remember because there is more of the sim to edit at the beginning , if your sim is posing with hands on her hips you will also have to click the background visible between her arm folds , leg folds, where ever, and delete these also or use the eraser tool. See below, some of my background was still barely visible. this will show up on the background, so click again with the wand tool and delete or manually delete it with the eraser tool, be careful to avoid your sim!
When you are happy with your edit, Remember to click outside the grey box to make the lines go away. Then click Edit>Copy and go back to the background with your main pasted sim and click Edit>Paste and you will see your full length sim there. You may need to Edit>Undo to go back to this sim to resize your sim to your liking or use the scale tool see below. Click that tool then drag the corner of the picture you want to scale/stretch on your template (*in this case, the floating layer of our full length sim.-you can also do this at the beginning if your headshot sim is much smaller than the template. But at the beginning if you zoom in enough in game while taking the picture, this shouldn't happen)
You will see if you click the picture, there was background left near the feet. Not a nice look you want so we click Edit> Undo, and go back to the sim and use the eraser tool to edit these bits out. When you are happy resize your picture to 512x512. It is better to so it this way, in the end, rather than starting out with these measurements because then you would have to shrink your sims very tiny. Also when starting out, make sure your template measurement matches in width and length. Don't start out with 700x650 because when you resize at the end it, your sim becomes becomes squashed. Use the tab button on your pc, and the arrow keys to zoom in on your sims. Good lighting and latest Custom content is paramount, also if you want good downloads. You could also get the Poseplayer, (but not necessary) to make your sim pose nice in pictures)
There you go :) remember to make sure you click outside the box to snap your sim into place. Repeat the process on other long shot sims with different outfits and paste in beside the sim, and so on.
You need to look around and google where to find extra info with gimp on cool extra features that the programme has to offer like lighting, enhancing the pics, sharpening etc, its not hard though, just brows around the Filters/colours/layers tabs etc and look up what each one does. You can copy a picture from the internet and use that as your background if you wish, Just make sure the sim has a brownish or grey background though because you will get outlines around the sim if the sims background is light but the background you want to paste it on is dark. The Decor one adds a border if you wish and the capital A in the toolbox, lets you add text to your picture.
And that's it! Like i said it's very basic, but its a start. It is not in-depth editing. I suggest to check Buckosnort's blog for tutorials on that. She has some neat tips and tricks for making amazing avatar pics.
If you get stuck don't be afraid to ask me anything, i will answer you the best i can. If you have an easier way to do things, or something i don't know or have left out please also let me know :) :D