It sounds like a problem with your OpenGL drivers to me. A screenshot might help explain what you're seeing.
Press F12 to take one. The .tga file is written to the game folder. Convert it to a .png file (preferable to avoid lossy jpeg compression given the issue) and then upload it to
http://imageshack.us/ or somewhere similar and just post the link (don't use the IMG tag to embed the image here).
If imageshack automatically resizes or reformats the image (I'm not sure if it will, but given that the file will probably be on the large size, it might) then you could try running the game at a lower resolution to take a smaller screenshot, or you might have to go with JPEG instead of PNG for the format.
Use
http://www.irfanview.com/ if you need something to convert between image formats.