As Savannah mentioned you should upload a .png or .gif version (the file you uploaded to Kickstarter is converted to .jpg, which doesn't support transparency, hence the white background). Best file to use is the 117x117 .gif you got in your package (It has the correct size for the forum, and supports transparency).

As said there is ImageShack, which is simple to use.
I think Photobucket looks more professional, but you need to register to use it.
If you still have trouble you could go to this thread: Setting Up A Custom Avatar.