in reply to merging two .gif or jpg (overlapping)

When you say "merge", do you mean making one image from two where the 'upper' image completely covers the lower image (like you had physically pasted one photo atop another); or are you looking to merge their alpha channels (as demo'd here) ?

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In Section Seekers of Perl Wisdom