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I've run into an issue where I was hitting the memory limit of Safari on the iPad (loading in very large image galleries) which was causing the browser to crash. (ouch!) It turns out that there is possibly no direct way to unload images from memory in this case, but you can force something to the same effect by changing the src of images you want to unload to point to a small (1x1px) image file. This will unload the real (large) image from memory, replacing it with the small one. Details at the link.