Estimated memory needed for ImageCreateFromJPEG
First I supposed simple width*height*bpp will be enough, it isn't though; there is some pretty big overhead.
$imageInfo = GetImageSize($imageFilename);
$memoryNeeded = Round(($imageInfo[0] * $imageInfo[1] * $imageInfo['bits'] * $imageInfo['channels'] / 8 + Pow(2, 16)) * 1.65);
With memory_limit enabled running out of memory causes script to crash; above written will tel you how much memory you're gonna need for creating an image-resource out of image-file. So in conjunction with Memory_Get_Usage() and Get_CFG_Var('memory_limit') you can avoid the mentioned ending. (Yet there won't be too many images blocked from processing that would still fit in the memory, as the results of this are pretty accurate.)
