You can do this from the command line (this started out being fairly straightforward and ended up being pretty convoluted so it's obviously not the best solution as far as clarity goes, maybe not for performance either): 
perl -pi'.orig' -e 'BEGIN {open(FH,"alt_tag_file"); @alltags = <FH>; c
+homp @alltags;}; @tags = @alltags unless(scalar(@tags)); s/(<img)([^>
+]+>)/@T = ($1,$2); $T[0].($T[1] =~ m,\balt=, ? "" : q{ alt="}.shift(@
+tags).q{"}).$T[1]/eg *.html *.htm *.shtml

[download]
All your original files will be renamed to be something like foo.html.orig (use perl -pi -e if you don't care about backups).

I might be misunderstanding the OP's desired solution (based on davido's response) but this solution assumes that you don't need to match up image names to alt tags (the alt tags are in the correct order in the alt tag file)
--Brian

In reply to Re: Slurping and s/// a group of files from a list by bpphillips
in thread Slurping and s/// a group of files from a list by Anonymous Monk

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