ZOOM is high resolution photo viewer that supports full resolution mode for very large images.

The default photos app of Windows Phone 7 and 8 shows up to 2048 x 2048 resolution images and the higher resolution will be downscaled. For example if you take a picture with Lumia 820 it generate 3552 x 2000 (7.1 MegaPixels) resolution HD photo but the default photos app shows the photo in 2048 x 1153 (2.3 MegaPixel) resolution.

ZOOM shows up to 20MP(5000 x 4000) of photos in low-memory(512MB) devices and shows up to 41MP(8000 x 5000) of photos in high-memory(1GB) devices without downscaling. Also ZOOM doesn't have limitation of the width or height of the photos so you can see 480 x 80000 size photo in full resolution mode.

If you are collecting HQ images larger than HD/4K/8K into your phone, you must have this app. Also if you are using high-end camera phone like Nokia Lumia 1020 or 1520, you can see greate quality of your pictures by ZOOM.
(PureView highres image is only supported on WP 8.1 or higher.)