They don’t need to be below 20kB in size.
I’ve got examples that work fine, and show as 82x82 images (on Clip Sport display), for a file as large as 83.6kB. I just scaled the images down (using ImageMagick v6.7) to 192x192 pixels, and file sizes shrunk (but not to below 20kB).
Note: Previously, I used the same technique for the Sansa Clip Zip, but scaled down to 128x128. When I saw the Clip Sport was available (with 128x128 OLED display, instead of 96x96 ), I changed to 192x192 , which still works fine on the Sansa Clip Zip.