Why would you want to encrypt a graphics file? Mostly to control who can view its contents.
You can invent a proprietary file format and that might slow a file format hack down for, say,
five or ten minutes. You could add a proprietary data compression scheme, possibly a twisted
variation of an already public algorithm. But there are so many people out there with nothing
better to do than hack at unknown data formats that your data would probably be exposed in little
time. But suppose we top off all this effort by encrypting the graphics file itself as we would an
ordinary text file. Would your data then be safe?