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Originally Posted by DaRkY
I think, that checksum calculating is not so easy  For example, checksum of 1 FF byte file is 3045181057, and checksum of 1 FE byte file is 1738016605.
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Sorry yes, I should have realised, that if the checksum is calculated by XORing, then my idea of just altering the spare space after the replacement GIF data won't work.. we would need inside info for that..
Can someone do a test... Alter one byte in the original GIF, and add one to it, then take another byte from the original GIF, and subtract one from it.. If when burnt to disc, the update goes OK, the the checksum is a simple totaling of the bytes in the file, and therefore my original idea could be used..
NavMan