Actually, you don't need to reinstall it to fix this if you know how to use the Windows Registry Editor. But if you don't know how to use Regedit, I'd recommend that you don't even read this post, because Regedit can very easily damage your system if you do something with it that you shouldn't be doing.
Anyway...
Navigate to this key:
HKEY_USERS\[whichever one you are]\Software\Paint.NET
And delete these values:
PaintDotNet.PngFileType
PaintDotNet.JpgFileType
Really, any key that looks like this:
PaintDotNet.[FileType]FileType
BUT DO NOT DELETE PaintDotNet.PdnFileType!!
Then, repeat for HKEY_CURRENT_USER\Software\Paint.NET.