Thanks for your suggestions. Just to be sure I also switched to default theme and tried the hole routine, but no look.
@acrylian: No, haven't looked at logs.
debug-1.log consists of the same two Notices over and over again (6MB of it), only the dates change:
`
{4547:Sat, 02 Mar 2013 22:14:53 GMT}
NOTICE: Undefined variable: galleryactive in /themes/frokost/header.php on line 359
include called from include (news.php [1])
from index.php [93]
{4551:Sat, 02 Mar 2013 22:14:59 GMT}
NOTICE: Undefined variable: videopresent in /themes/frokost/album.php on line 154
include called from index.php [93]
`
debug.log is pretty much the same. I guess I should look into those but it seems they're unrelated. Are there other logs I could look into? security.log didn't reveal anything either.
File too large or corrupt seems unlikely as after switching to the new one and back the old watermark stopped working, too. File sizes are pretty much equal.
@sbillard cleaned cache again and again. Downloaded image from cache, no watermark on them. Supplied ones don't show either.