Update image data in existing imshow

Solved it myself!

So the image data that imshow displays is the exact data I should be changing:

data = rand(RGB, n, n)
g = imshow(data)
data[1] = RGB(1,1,1) # update the image data

Now nothing has yet been updated in the window, so I need to fake an update event. I do this by:

zr = g["roi"]["zoomregion"]
push!(zr, value(zr).currentview)

and it works!