One side note: Despite the {$minenumsize 4} defined in ImagingOptions.inc, TImageFormat is *still* one byte in size! I had a nasty run-in with AVs when I redefined TImageData in my code, with Format having a type of "dword" (now corrected to "byte").
Thanks for bringing this up. This is most probably cause of crashes of Vampyre compiled to DLL and called from C. I've just tried moving {$minenumsize} to other places and it seems that it gets reset later by setting {$mode}. I've corrected this and updated the code in SVN repository - you can change it back to "dword" if you use this new code.