Some other things to check on your side just to make sure, as Ruth said 500k should be no problem for the library, but I think even my computer would freak out at 500M
1. Are you on dial up? If so, you are probably timing out because of your ISP and/or computer settings.
2. Have you cleared your cookies and temp files lately? A full batch of either will often slow uploading.
3. Have you checked for spyware/adware? Either one can also interefere with uploading.
4. Verify the file is indeed a valid JPG and make sure that its dimensions do not exceed 2500x2500 pixels. Larger file sizes in Kb sometimes result in larger than 2500x2500 pixels, and the library will reject anything over that pixel size.
Hope these help!