Im kinda new to SWC and I wanted to change my avatar. I tried 2 different avatars to see if they would show up good enough. (They didn't). And when I finally got the right size and went to upload it. I got an error message 'error 2 file may be too big' or something along those lines.

Download your desired avatar as a file on your computer.
Resize it to 100x100, cropping/etc as required.
Save it as a PNG or a JPEG.

Check the file size of the resulting image, if it is > 100KB then you need to make it smaller before you can upload it.