As far as I know, such a version of PS doesn't exist.
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There is no portable version of Photoshop, not one that can be launched from a Flash drive. Photoshop's TOS does let you use it on one desktop and one laptop, provided they're not being used at the same time. That's as portable as it gets.
GIMP does have a Flash drive version, and it's free. Check out portableapps.com.