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Reading the text on the page with license I think I see where the problem lies. What Nikola Gruev says in Bulgarian is that ""Only the pictures used in Wikipedia are under GFDL - for all the others you'd need a separate permission. 
Here is a translation of his third letter form 8th November 2004(the first is from 2nd NOvember 2004 and the second from the 4th November) 
""For the needs of Wikipedia (the Bulgarian or any other version) I hereby give you permission to use all the pictures from my galleries found suitable by your team given the conditions you mentioned. This accounts for all images I have uploaded as well as all images I upload in the future. I wish you luck in this difficult, but noble initiative.""  T "