I have noticed that some people here seem to be having problems with uploading and cropping images.
I thought I would just post this to possibly help anyone that is having problems of this nature.
If you are having problems and getting error messages after you have resized the crop box and saved it, check to make sure you have the GD libraries installed on your server.
apt-get install php5-gd
sudo apt-get update
sudo apt-get install php5-gd
Maybe this information should be added to the "usage" tab in the plugin info page?