Another one I would have spent hours on was solved in two minutes by the wonder that is the internet. More specifically Chris Garrett over at ASPAlliance explains the solution to the png generic error in GDI+ exception .

The short story is you can’t use the Bitmap Save() method with a non-seekable stream. So, instead of just doing this:

img.Save(context.Response.OutputStream, ImageFormat.Png);

You have to do something like this:

MemoryStream memoryStream = new MemoryStream();
img.Save(memoryStream, ImageFormat.Png);
memoryStream.WriteTo(context.Response.OutputStream);

Cheers Chris, you saved me hours of work!