Image sizes
Design and Skin Customization
I've been testing that on url images like
/blog/image/blog_image/4/xxlarge/crop=9:18
So far it holds up for blog images of all sizes and aspect ratios and original upload aspect ratio.
The cache problem I was seeing seems to be firefox.
After emptying the jr image cache, ff cache will show a previous version for the same url. You can get around that by changing the image id, size or aspect ratio in the url each time so its always a new url; crop=122:133 crop=122:136 crop=1342:1396 etc