You only need the last modified date in the header to see if the image has *changed*. Of course you can just touch the file to change the last modified date in which case the image is the same and only the M time has changed. The top of image files contains header info about the image. For example the size info appears near the top. With GIFs it is always 6 bytes in. With PNGs it is 16 bytes in after the IHDR token. JPEGs are less convenient. For example the size info is a variable distance in with 163,194,612,746 bytes being common offsets. The point being 500 bytes is not enough of a JPEG. You should use Image::Info which can extract lots of different features. It will barf on partial == corrupt images so you will probably need to hack it a bit.

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