Open the program:
Find screenshots ( My Documents/Electronic Arts/ The sims 3/screenshots)
Drag your captures to the center and they will appear at the top of the workspace.
You can move the order of the pictures by dragging at the top.
Change the speed of the gif and effects also.
Right click on the capture in the above are and you can delete/ move/ etc.
Once you have your captures arranged and your speed and effects set you can save your gif.
Upload to your chosen host site as normal.
I am sure there are more sophisticated ways of making gifs ….this is just a way I have figured out on my own. Hope it is helpful.