Hi,
I'm not yet convinced as you are that it is something about the file itself causing the error. If you post links where I can download the files if it produces an error for me when I upload it then I will agree with your diagnosis but not until then. The fact that the problem started suddenly after working fine for a while indicates that something changed about your environment and not likely a problem in the code.
Even if it does turn out to be a file specific issue, the error happens in the constructor of the .NET framework Bitmap class and therefore is most likely outside the scope of what I could fix. If it was caused by mojoPortal code that would be a different issue.
For now I still think it is a some kind of permission or configuration issue on your hosting environment or server. Or it is timing out or somehow not uploading the entire file and therefore corrupting the file so that it causes this error.
You never mentioned what version of mojoPortal you are using.
Best,
Joe