If you don't like the built-in photo viewer in the Photos app in iOS, RotoView has a free solution for magnifying your photos and easily scrolling around with gestures and phone movements.
A double-tap on the screen zooms in. You can scroll by moving a finger and rotate an image by moving the phone. There's also something called "Throw and Glide." The Throw gesture rapidly scrolls enlarged photos in response to the way you tilt your phone at a rate much faster than the standard flick touch gesture. The Glide gesture follows the Throw gesture automatically with slower and more precise scrolling. The RotoView interface works well with touch gestures, providing a smooth transition between rotation and drag.
Some of the moves are very complex, and certainly outstrip what you can do with a stock iOS photo viewer. Settings are also complete, and you can adjust the accelerometer and rotation thresholds, as well as the amount that an image rotates. Thankfully, you can also turn off the sound effects, which sound pretty obnoxious...
RotoView Photo Magnifier
If you have a lot of photos on your iPhone or iPad, and often find you need to examine them in more detail, the RotoView app is for you. The price of free is just right. Prepare to spend some time figuring out the gestures, and soon they will be second nature.
RotoView Photo Magnifier is not a universal app, but runs on iPads at iPhone resolution or scaled up. The app requires iOS 6 or greater, and it's optimized for the iPhone 5.