How can I compare two pictures from the camera and tell if there are enough differences to detect movement?
It seems to me what you are looking for is background subtraction technique. With noisy images and changing lighting conditions it could be nontrivial. The current state of the art technique for this is the low-rank matrix representation, but it require not two but many (~dozen) images. Some heavy duty math follow:
Each image considered as  a vector of pixels, vectors combined into matrix and this matrix is decomposed into low-rank matrix and remnant. Low-rank matrix columns are backgrounds and remnant is moving objects. There are some open-sourced implementations, but only for factorization itself, not complete image pipeline IIRC

Here is a paper and code for matrix factorization
http://www.ece.rice.edu/~aew2/sparcs.html

Here is overview from CS blog and link to other code:

https://sites.google.com/site/igorcarron2/matrixfactorizations

http://nuit-blanche.blogspot.com/search/label/MF

Survey of some other techniques:
http://www.vis.uni-stuttgart.de/uploads/tx_vispublications/Brutzer2011-2.pdf