The issue is that Badspot has expressed dislike of this idea previously, most likely due to bandwidth reasons (could be pretty bad for the blockland.us server if a ton of people use the add-on).
Which is why you just make it obey proper cache control headers, assuming that the server actually sends those. If the add-on obeys the HTTP spec for cache control and the server doesn't that's his problem, not ours.
It'd be really nice if the server didn't make partially-updated images available, frankly. It'd be even nicer if it just uploaded the whole image from your server if more than, say, 1000 changes (placement, recolor, removal of bricks) have been made since the last upload. Or if you could manually upload a screenshot if you're verifiably the host of the server and you take the screenshot in-game, to prevent false screenshots.