From a news:comp.lang.tcl posting by Berry Kercheval < mailto:berry@kerch.com >:

And this works for simple GIF files:

 proc gifsize {name} {
    set f [open $name r]
    fconfigure $f -translation binary
    # read GIF signature -- check that this is 
    # either GIF87a or GIF89a 
    set sig [read $f 6]
    switch $sig {
        "GIF87a" -
        "GIF89a" {
            # do nothing
        }
        default {
            error "$f is not a GIF file"
        }
    }
    # read "logical screen size", this is USUALLY the image size too.
    # interpreting the rest of the GIF specification is left as an exercise
    binary scan [read $f 2] s wid
    binary scan [read $f 2] s hgt

    return [list $wid $hgt]
 }
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See also [Reading JPEG image dimensions] and [Reading PNG image dimensions].
[Category Graphics]