The path was:
C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Microsoft\Media Player\Art Cache\LocalMLS
(in Czech version the Art Cache is called Mezipaměť grafiky, so there is a high chance that in different language versions of the Windows this folder is translated in the language of your Windows).
Well I had there more than 100 thousand of files totally consuming more than 3GB... what is rather strange though is a fact, that I am actually not using Windows Media Player at all, so why the hell does Windows create this kind of database of album covers?
So I deleted all the jpg files and I have free space again...
P. S. Apparently there is another location of the same:
C:\Documents and Settings\
But here it was only couple of megabytes
UPDATE!!! So the interesting thing is that Windows 7 is making these files again and again and again... Sooo the trick is to disable this. I found this as a solution, have to check first if this really works.
http://www.mydigitallife.info/how-to-disable-windows-7-media-player-network-sharing-service-wmpnetwk/