When did it become a verb?
A few years ago Photoshop was a software tool used by photographers and designers. Now it's become a verb used by the general public. Adobe Systems Inc., the makers of Photoshop, aren't happy with this.
From the Adobe website:
The Photoshop trademark must never be used as a common verb or as a noun. The Photoshop trademark should always be capitalized and should never be used in possessive form or as a slang term...
The following examples illustrate these rules: Trademarks are not verbs.
Correct: The image was enhanced using Adobe® Photoshop® software.
Incorrect: The image was photoshopped.
Good luck with that, Adobe.
By the way, do you know how I found that reference? I googled it. ;-)