Watermark won't work

I tried the watermark feature and uploaded my watermark in several formats, but it just won't work. It won't show any image anymore, and I get this error message:

warning: imagecopymerge(): supplied argument is not a valid Image resource in /var/www/auweia.org/rufzeichen-test/sites/all/modules/imagecache_effects/imageapi.inc on line 90.