UPDATE: Problem solved!
I installed the Community Builder and made it work, Gallery works too, I can upload and display images.
The only problem is with videos - as soon as I upload a video, the error message appears that the file is too large and that the maximum size is 1024 Kb. I've searched everything but I just can't find a setting for the size of a video upload. Has anybody experience with that? Where can I adjust the size? I have no problems with photos, there I can upload 8MB without problems.
sorry for my bad English
Luky