Click on "choose file." That will bring up a window with places on your computer where you've stored files and folders and jpegs and stuff. There will be a little "open" command box at the bottom right of the window.
Click on the image you want, then click "open." It will then return you to the Picasion screen, showing what you just selected, and allowing you to add more..
They have three "choose file" buttons showing, but if you keep clicking on "add one more picture," you can go up to ten.
Once you're done adding images, click on "create animation," and the site will do the rest. It will also ask if you want to save to your computer. You'll want to do this so you can upload it to your photo hosting site, like Photobucket.
One suggestion, when you complete more than ten group builds. Use the GIF you created for builds one through ten as one image: click on that GIF so it adds the whole thing as one file when you "choose file. " Then you can add the file image for group build number eleven.
Hope I made that clear 
This will avoid running into the problem Bish mentioned.