Some of the ones that display poorly on the kobo (in Calibre they show fine) include:
305x500 (doesn't even display the cover)
Most of these were got with the download cover feature, usually from Amazon. I tried Google searches as well and just dragged some covers from the browser into Calibre on the cover art frame and it changed in Calibre but it still seems to be the wrong size.
Would like to know what the ideal or recommended dimensions are for cover art on the Kobo, either in the Kobo app or Aldiko (or just generally).