QuickPic is undoubtedly I think the best picture app for Android.
I see people still use the awful and painstakingly slow stock Android gallery which does not even let you zoom in on high-resolution pictures. Due to the 3D effects of the gallery, it has to go through your memory card and cache all the images so it can enable these 3D effects - if you have over 1000 pictures you will be there forever waiting for it.
I admit, these effects are cool and very different from other picture apps you will find - but some people (like me) don't really care about these effects and just want to view our pictures.
QuickPic is a very simple gallery app for Android which simply displays your pictures in a tile format, and it is bloody quick. It will read all the images on your SD card in no time and will enable you to zoom into pictures, viewing all the little details.
There are also some great features, like hiding folders or pictures with passwords, ability to play GIFs, ability to view videos, crop, shrink, set wallpaper and more.
It's got a very high rating of 4.7 out of 5 on the Android Market and has been downloaded over 1 million times, so it's doing something right.