I don't have a working link yet, everything works local only :-(
The system means that picture's are resiced so that they can fit in a box. If you've got a picture of 200 widht/100height and you've difined the picture to be of size 100 100,
what the system wil do is the following:
it will find that width is the biggest, calculate the ratio as being 0.5 and then multiply the picture sizes with it.
The new widht wil become 100, and the height 50
the picture now fits in the 100x100 box and stil keeps its ratio!