its probable easiest to build up to this. Suppose i fix the dimensions at x and y. Lets find the number of JPEGs that can be made.

There are xy pixels, and each one can take 256^3 values. So the number is

Step 2

Now lets vary one of the dimensions (x), while pretending the other(y) is fixed at some value. We can just add up our formula for all the possible values of x

Combinations if height is fixed at y :

Step 3

Now lets vary the other dimension. We can just add up the previous formula for all the possible values of y.