I`m using Lumia 900 and WP for a few days. Generally this is a great phone and WP is very nice too, but i`ve noticed few extremally annoying issues. The problem that you are talking about is one of them. You can see it not only on pictures that where made by the camera, but also on pictures that were downloaded from internet. This is something with resolution, probably WP cannot fit high resolution pictures to its resolution. But the weirdest thing is that like you said, slighty zoom is solving the problem.
BTW. There is an option in Zune that when you are sync pictures from your PC, it can automaticly convert these pictures to Lumia 900 resolution, but this is a bad solution becouse your pictures becoming probably to 800x480 resolution on our device. It`s absolutely unacceptable.
Yesterday I downloaded new WP 7.8 and I was hoping that they finally solved some issues (especially this one) but they didn`t... Nothing have changed with other issues too... It means that probably I will sell my Lumia 900, this is a great phone, but such things like for example this gallery issue are unacceptable for me.