EDIT - Found the solution, I was editing the ones in my documents rather than the title directory in the SteamApps folder.
Is anyone else getting this problem? I tried a google search but it doesn't seem to be common. I use Paint.net and have done for many years, I don't know how to use PhotoShop. When I open one of them with Windows Photo Viewer it says that the file could either corrupted or too large. This is what it spits out when I try to edit any of the images in Paint.net.
System.ArgumentException: Parameter is not valid.
at System.Drawing.Image.FromStream(Stream stream, Boolean useEmbeddedColorManagement, Boolean validateImageData)
at PaintDotNet.Data.PngFileType.OnLoad(Stream input) in d:\src\pdn\paintdotnet\src\PaintDotNet\Data\PngFileType.cs:line 118
at PaintDotNet.FileType.Load(Stream input) in d:\src\pdn\paintdotnet\src\Data\FileType.cs:line 477
at PaintDotNet.Functional.Func.Eval[T1,TRet](Func`2 f, T1 arg1) in d:\src\pdn\paintdotnet\src\Base\Functional\Func.cs:line 158
Specs: Windows 8.1 64bit | Nvidia GTX 650 | i5 Quad Core 4th Generation | 4GB DDR3 RAM
