The Nook Tablet is adequate for viewing pictures and videos, but I consider the Fire completely unusable for viewing images.
Much of a tablet's appeal rests in the apps you can run on it--not just just how many are available, but how good they are and how attractive they look on the device.
Since you're limited to the apps available in the small app stores run by Amazon and Barnes & Noble, neither tablet is a good choice if you want the hottest Android apps right away. You have no way of knowing whether cool apps will be available for either of these devices.
And since both tablets appear to lack capabilities such as multitasking and (in the Nook Tablet's case) standard menu overlays, some cool present and future apps may not run on either device.
Amazon and Barnes & Noble are trying to enlarge their app stores' holdings, but neither will come close to the number of apps that Google Market offers for Android 2.3 or that Apple offers for iOS. At launch, Amazon said that it had more than 8500 apps for the Kindle Fire, and it continues to add apps daily. Barnes & Noble says that it is increasing its collection and that it expects to have about 1000 apps by year's end.
As for how well apps work on the devices, the Nook Tablet shines in this area. Many of the apps I downloaded seemed well adapted to run on the tablet's 7-inch screen instead of looking like blown-up versions of standard Android phone apps. Barnes & Noble says that it works with its developers to optimize apps for the Nook Tablet, removing references to features that the tablet lacks, such as a camera or a GPS connection. The company also says that, in curating what apps go into its store, it tries to filter out bad-looking apps; the downside of this policy is that you end up with dramatically fewer choices than are available elsewhere.
To judge from the apps I downloaded, Amazon seems to take less care to ensure that the apps for its tablet work well and look good on a 7-inch screen. I say this despite learning from a company spokesperson that "we test each app to make sure it works great on Fire." Though I found fun games like Airport Mania: First Flight (not available for the Nook Tablet) at Amazon's app store, the graphics looked fuzzy, as though they'd been blown up to fit the 7-inch display. With apps available for both Kindle Fire and Nook Tablet—such as Pandora and Quickoffice Pro--the Nook Tablet versions typically had better layouts and graphics. Even the Nook Tablet's Hulu Plus and Netflix apps looked better than the Kindle Fire's.