Flexible image geometry

This morning's pull request, unfortunately, did have issues, as I found out. 

The ```c``` modifier at the end of an imagemagick resizing string crops without resizing first, which is usually not what people want. The ```#``` modifier, however, does what we want: It first resizes so that as much as possible is seen from the picture when using ```@size``` as a mask, and then crops. 

I also renamed the string generation methods somewhat to reflect what they're used for. 

I have not found a solution for getting rid of ```upsample``` or ```normalize_sizes```.