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