[edit: Bad internetconnection -> two times same post]

The constructor int, int, PixelFormat throws an exception when the format does not start with Format. On the other hand according to the MSDNA the constructor int, int invokes the other one with Format32bppArgb so it shouldnt throw an ArgumentException.

You might wanna debug the bitmap constructor using the .NET source code so you can ispect where the exception is actually thrown. Here is a link how this can be done:
http://msdn.microsoft.com/en-us/library/cc667410.aspx