Someone said you could only use it in the photo store, however the Amazon.com page does not say that, it says its a Amazon gift certficate, doesn't say anything about limiting where you can spend it.
"For a limited time, new members who upload at least 25 images and order their 25 free prints can earn a $25 Amazon gift certificate. This offer expires on June 30, 2001, is limited to one gift certificate per customer, and is valid for new members only. "
Although with my latest experience with amazon and their gift certificates, they can be extremely picky about what gift certficates you use. Apparently some are considered "promotional" gift certificates and aren't combineable. (even though the stupid certificate doesn't mention anything about "promotional" on it).