Set the memory limit higher if you can. Also realize that the dimensions are more important than the file size (as the file size is for a compressed image). When you open an image in GD, every pixel gets 3-4 bytes allocated to it, RGB and possibly A. Thus, your 4912px x 3264px image needs to use 48,098,304 to 64,131,072 bytes of memory, plus there is overhead and any other memory your script is using.