[SOLVED] Error: Unable to open image /chroot/tmp/... on change canvas.

Ahhhh, the end-user confesses that it was a *.png image that she just changed by file-manager rename to *.jpg.

That explains all. I didn't expect that kind op stupidity, but maybe you can look or you can generate a better error message when this happens.

Kind regards,
Nico

Yup, changing the extension will corrupt it. No, the message won't be changed as it's stating exactly what it can't do (open the image), which could be any number of reasons (not just extension change).