When I am setting the image size, I have the option of setting the size of "width", "height", "longest side", "shortest side".
I think it would be better to instead just have the options of "max height" and "max width", where you have to set at least one of them.
This covers the options that are there now, but additionally gives the possibility to set an area that the image has to fit within. Let's say for instance that I want all pics to be 700px wide, which I can easily set using the current system. But sometimes I have really tall pics, and I don't want them to be restricted by the width of 700px, but by a height of say 1000px instead. This is not possible with the current system, as far as I can see.
I would imagine this to be fairly easy to implement as well.