probably not a resolution thing, resolution either works or it doesn't (you get black screen and or bumped to desktop) or it distorts your view (wrong aspect) but it doesn't add any artifacts
I'd try re-downloading the client to different folder (can't rely on overwriting files), total fresh start using autopatcher or full download (could be map file corruption)
big clue and weird thing is that screenshots don't have the artifact (might be something related to directX), re-downloading might not solve it on second thought but always worth a shot anyway
more info for experts might be if the artifact moves with you or if it is part of ground and if you get it on PI map only, did you have it from the very start or it showed up one day... etc