It is something we have experimented with. If you look near the foot of image.aspx, you can see this code:
'In theory this lets us output thumbnails
'in the original format, but PNGs don't
'seem to work reliably
'Select Case FInfo.Extension.ToUpper
' Case ".PNG"
' bmpImage.Save(Response.OutputStream, ImageFormat.Png)
' Case ".GIF"
' bmpImage.Save(Response.OutputStream, ImageFormat.Gif)
' Case Else
' bmpImage.Save(Response.OutputStream, ImageFormat.Jpeg)
'End Select
We didn't get it working reliably, but you can experiment with this. The code is open for this reason; there are also various ways to handle images, so it may be possible that rewriting other parts of this file to use alternative image and graphics methods may resolve this.