I already had unique images for each one. 1080p and 720p both display the hd image 290x218, both seemed to appear about the same quality. Only SD Roku displayed the 214x144. The different looking fhd 540x405 image was never shown. This is a side-loaded app, so maybe it works different with a private channel or a published channel.
I did another test, which was to make a big shape in-game, exactly 800x320 (based on a pic I already had). When Roku is 720p, the in-game shape (red block) is EXACTLY 800x320, no scaling for overscan or anything. When Roku is 1080p, the shape is exactly 1200x480 (50% larger each way), which is roScreen scaling the 720p game up to 1080p perfectly. So in-game there is no issue with HD overscan from either the HDMI or the elgato device. I also double confirmed this result with direct Roku screenshots in png mode.
(Interestingly, on this computer monitor, I found a setting for the HDMI input to act either as "Video" or "PC" - when I put it on "Video" it slightly zooms in the screen, cutting off the edges a little bit, creating overscan! Of course I set that junk back to "PC" where it will remain forever - where it was before.)
Back on subject...
So in-game there is no overscan happening at all. Is it possible there is some overscan happening only on the Roku home page? Maybe now you want to spill the beans how you measured 290x218?
Anybody else have a way to capture a screenshot of the home page in 1080p and measure some pixels as they appear on-screen? See if the channel pictures are 290x218 as they are supposed to be, or something more like 295x223 as I'm measuring.
Maybe there is some crapification of side-loaded icon image files that doesn't happen the same way with private/public channels. Maybe they get squished, prodded, translated to jpg, enlarged, pushed into a box and shrunk down again, and compressed to boot.