one of the documented procedures in this installation of the ACS
ad_image_size   graphics_url
What it does:
Returns Tcl list of WIDTH and HEIGHT of image, works for both JPEG and GIF. We need our own proc because AOLserver is stupid and has separate API calls for JPEG and GIF.
Defined in: /web/philip/tcl/ad-sidegraphics.tcl

Source code:

    # call ns_gifsize or ns_jpegsize, as appropriate
    if [string match "http://*" [string tolower $graphics_url]] {
	# this is a image on a foreign server, we won't be able to 
	# figure out its size 
	return ""
    set what_aolserver_told_us ""
    set full_filename "[ns_info pageroot]$graphics_url"
    set guessed_type [ns_guesstype $full_filename]
    if { $guessed_type == "image/jpeg" } {
	catch { set what_aolserver_told_us [ns_jpegsize $full_filename] }
    } elseif { $guessed_type == "image/gif" } {
	catch { set what_aolserver_told_us [ns_gifsize $full_filename] }
    return $what_aolserver_told_us