3.) Open Gimp. Go to File----> Open. Select the userbar that you made on that site. By now you should have something like this
4.) Go to a place to get a sprite, Do NOT use a animated sprite for this tutorial, and save the image of the sprite you want to use. Now go to File----> Open As Layers(CTRL+ALT+O shortcut)
5.) Shift+T to move the sprite where you want it to be, DON'T resize it. The sprite is probably cut off. So go to Image----> Canvas Size and change it to your needs. Then go to the first layer(the one with the userbar, not the sprite) down to the bottom and do the same with the sprite. Now, your probably seeing a checkered background like http://i.imgur.com/JLdD2jy.jpg, that doesn't mean anything.
6.) If you just click "Save" it will save as a Gimp file, which sites like Imgur don't support. Click File---> Export(Or Shift+CTRL+E) and save it as a png for highest quality ANDIMPORTANT PLEASE READ! MAKE SURE THE MODE IS RGB! Go to image->Mode-> and change it to RGB if it is not in it!
7.) Now upload it to a image sharing site of your choice. I prefer Imgur as it's quick and easy, and you don't need a account.
If you have any questions, feel free to ask me and I'll answer to the best of my ability ^.^
This won't stretch it. I think what your doing is resizing the sprite layer. Go to the userbar layer, the one without the sprite on it. Then go to Image-> Canvas Size and change it. PM me if that doesn't work.