this is not a task where a CAT tool can be applied.
First of all, gif files are graphic files, and they need to edited or recreated using a graphic software.
All you could do, as a translator, is to provide the text for a graphic designer to produce the corresponding localized buttons, unless you have the necessary graphic skills...
Secondly, there are probably no repetitions and there is no advantage in using a translation memory for their textual content.
PS: I understand that you only received HTML files containing the buttons.
If you received, into those HTML files, the website content too, you can deal with them using TagEditor. HTML is a tagged format and TagEditor can process them without the need for any DTD file.
[Edited at 2008-03-25 19:42]
Thanks for your answer. That was what I thought too - just wanted to be sure.
Yes I received all the html files with the website content. I will of course translate them with Tageditor. The point is of course that the text of the buttons will probably be in the other html files as well - that is why I wanted to use Trados. And I just hoped there might be a way to save me from opening every gif file and tip the text (or its translation in another file).