Bottom Silk Problems


I have been trying to no avail to get a rear silk image onto my board. I have uploaded various files in many formats, but the bulk of them get an unknown condition after I go though the upload process. Occasionally a file will upload successfully, but will be substantially smaller than my board. Any help would be much appreciated as a rear silk is a very cool way to add some style to a board.

What is the preferred image format?
Does the image size relate at all to the size of the board?
Should the rear silk be a mirror image of what I actually want on the board?
Is there a tutorial or detailed instructions about this process?