Color depth and resolution will _tend_ to be off 2 types
implicit or provided:

implicit: a GIF image is 72 dpi, with an 8 bit based palette attached
(this does not mean 256 colors)
provided: a JPEG or TIFF image should provide the info in it's
 header.

Then you have image formats that in fact can contain _more_
than one image. A TIFF, EPSF can contained hi-res images with
a lo-res display image. A PICT file can hide hi-res info in
PICCOMMENTS. A vectorial image has no resolution until rasterised
and even color can vary!

There is I believe a very comprehensive handbook on graphics
formats that someone has previously mentionned on this list
and hopefully they'll point you in that direction as this is
too complicated to be treated in 3 lines.

jonathan
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