I was creeping on Pokemon Trainer Touko's page and ran into your conversation...not to be creepy, but:
1. Flatten image (Right click on layers, flatten)
2. Filters menu
3. Decor submenu
4. Round Corners option
If you look at my sig, that's how I did it in GIMP (btw stands for GNU Image Manipulation Program). Super easy, keeps you from having to deal with uploading it somewhere else after you edit it in GIMP, and yeah. :3