Are you attempting to upload a new thumbnail when you see this behavior? I can't quite follow what it is you have edited before clicking the Save button. I've had the same behavior as you describe when I tried to update the application thumbnail by saving the changes. The issue behind my problem was that the image size of my file for upload was too big. The Portal beta release did not have a file upload size restriction, but the 10.2 production release does; most likely that hasn't changed at 10.2.1 also. Try reducing the pixel resolution of your image to something smaller, like 199 x 100 (this is an example, as you'll want to maintain your image aspect ratio of course) and try the upload again. I'm not sure if this helps you or not, but hopefully it does.