Sorry, I never even considered anybody would try to save such line-art as JPGs. (To be honest, I never even considered saving the image, anyway. I would have it alongside DF, use it purely as reference and then close it unsaved, however, given that some people would want to...)
For saving, PNGs FTW, though (or, for less colour depth, GIF or even for monochrome a compressed form of bitmap or other). Older versions of Windows may or may not have Paint able to save as that, so you'd then be falling back to the GIF (yeah, I know about the historic patent issue!) or BMP formats. The latter being a totally inefficient save format, and often (not always!) uncompressed, but then who's bothered about a few MB here or there, these days..
But never JPG.
My camera saves in a JPEG format (JPEG/JFIF, to be more precise) and wish I could afford to switch it to save as raw, but then I couldn't take as many photos before having to change memory cards), but after editing the pictures (usually lowering the final resolution, so losing most of the artefacts while doing that) I'd almost exclusively save them as PNGs if they were just for viewing.
Despite there being a better compression rate in under the JPEG system (and tunable in that regard) and that real-life images have smooth transitions that work very well with the DCT algorithm without it being so obvious.
So, back to building round buildings, are then taking the above into consideration, perhaps I should take a picture of something, down-convert it into a lower-resolution 16-colour dithered equivalent image and then carefully build a fort resembling that layout of pixels, with various walls/floors/etc being made of cinnabar, cobaltite, kimberlite, olivine, etc, in order to get something that (at least when zoomed out) looks remarkably like the image...