Apparently the array img has more than two dimensions, so writing its transpose is not well-defined. I would guess its third dimension represents its three colors, red, green, and blue.
Whereas a two-dimensional array has only one other way to rearrange its two dimensions, a three-dimensional array, for example, has five other ways. The matlab command for doing this is appropriately called 'permute', there being as many ways of arranging n dimensions as there are permutations on the numbers from 1 to n. You will have to decide just what it is you wish to happen to the dimensions of img in your fwrite command.
