I think most of the agency license agreements specifically prohibit using anything in a logo, so at a minimum it's a license violation. When someone uploaded works at Dreamstime that were a composite of one of my images and one of their own, it was treated as a copyright infringement and IS worked to get the material removed. I also believe that sometimes there are legal arguments over the boundaries of derivative works and what infringes.
Are you looking to stop them doing this or get paid appropriately for it? If it's the latter, how about proposing an RM license that gives them the right to do what they've done in return for some cash for you?
Thanks for your opinion.
You're correct, if one of my images was used in a composite and was offered again for sale, the agencies consider that copyright infringement.
I would think it is considered copyright infringement because not all parts of the composite are free from third party rights.
Now, would it be wrong to assume that such involvement of a third party would make it impossible to use it as a logo? I though a logo by definition has to be free from third party's rights...

I don't want to release any further info as I'm still in the middle of this but once this is taken care of I'll keep you posted.