My link, the one in blue towards the bottom of my post, does work. My comment that didn't show up was my attempt to copy that blog post to show you, but you can just click on it and see.
Let's say you use the numbers 500 and 800; if the width is lengthier than the height, then you put the width as 800 and the height at 500. But follow the spacing in the a2k blog link, and put your photo url in between, as the example shows. Photo urls usually end in jpg