Paperclip/ImageMagick image geometry specifiers
This can be a pain to find so here is the quick access version:
<width>x<height><specifier>
% Interpret width and height as a percentage of the current size.
! Resize to width and height exactly, loosing original aspect ratio.
< Resize only if the image is smaller than the geometry specification.
> Resize only if the image is greater than the geometry specification.