error when applying "canvas"

Hi,

After trying to set a "Define Canvas" to simply add a black border to my images, i get the following error message:
warning: imagecolorsforindex(): supplied argument is not a valid Image resource in /home/exoteran/public_html/lduros/sites/all/modules/imagecache_actions/watermark.inc on line 50.

The images are still displayed, despite the error, but do not have a canva.

THanks for your help.