you can always try to manually change the file names suffixes for different sizes.
s small square 75x75
q large square 150x150
t thumbnail, 100 on longest side
m small, 240 on longest side
n small, 320 on longest side
- medium, 500 on longest side (no suffix)
z medium 640, 640 on longest side
c medium 800, 800 on longest side†
b large, 1024 on longest side
o original image, either a jpg, gif or png, depending on source format