Question: max of an Array.

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.

I need to find a starting value(t1)  that is the avg(max,min), where max and min come from the Array.  After getting the starting value (t1) then i want to divide the set of number into an uppper and lower half  the compute the means for the upper and lower halves, and thave t2=avg(meanUpper, meanLower) and repeat this process till I get abs(t_{n+1} -t_n) < cut off value.

My biggest challenge is first finding the max and min of the Array which is the image.