I heard Google employees say that the important part of SEO for images is not the file name but the alt attribute.
And as the alt attribute appears only in the HTML code of the relevant page and not in the file path, it would seem to me that just translating the alt attribute would be enough and most importantly wouldn't oblige you to duplicate all the images.
So if you have a German and an English version, both versions would point to image.jpg, but the tag on the German version would read alt="ein in-house SEO auf der Suche nach einer Lösung" and on the English version it'd read alt="an in-house SEO looking for a solution".
Also mind that Google has repeatedly warned not to do key word stuffing also in image file names and alt tags.
[edited by: seidelbast at 3:58 pm (utc) on Oct 4, 2012]
You are welcome ..:) didn't want to take the risk that you might not be around for a half hour or so, and thus get back too late to edit..( Mod or admin can tidy up the thread later, if they get the time )