I checked the forums and only this suggestion didn't give an error message, but the results is not helping me out much.

max(flatten(Img_pigeon_HWT));

results in

flatten(Vector(4, {(1) = ` 1..512 x 1..768 `*Array, (2) = `Data Type: `*float[8], (3) = `Storage: `*rectangular, (4) = `Order: `*C_order}))

The purpose of all this is to do some edge detection upon an image.