Cannot process image: The background format is invalid.

[davidsotaabdd]

This happens because (at least on newer CMS versions) the colorpicker widget forces the color value to be uppercase and the validation rule for the background color is the following regex:

/^#([a-f0-9]{3}){1,2}$/

which need only be changed to one of:

/^#([a-fA-F0-9]{3}){1,2}$/ or /^#([a-f0-9]{3}){1,2}$/i

in the file classes/Resizer.php