Ah, all right, that declares the problem. What can I do about the brushes wich are hotlinked from this forum but are no png, gif, JPEG, bmp or gbr? Just look for other nice brushes or can I convert them some how?
Edit:
I've downloaded abrViewer.Net and it seems to work. Now I'm going to find out how it exactly works so I can use all those wicked brushes in my creations!