The following is based on a google search of:
photoshop "assertion has failed"
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a. Did this problem just recently start happening, i.e., you used to do this without error?
b. Photoshop or ImageReady? What version?
See:
http://forums.pcworld.co.nz/archive/...p/t-40883.html
Note... this thread a couple years old an mucking wih one's registry can be hazardous to your PC's health if you don't know what you're doing.
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If images are not RGB, that might be it per:
See:
http://support.adobe.com/devsup/devs...docs/52489.htm