This issue has been present in every past build of the webclient that I've tested, including the latest 507.1284 build. There is no problem when playing via the Dream Seeker.
If you place an image on a player's mob, that image is rendered in a very jerky way when moving.
Numbered Steps to Reproduce Problem:
Load this demo into the webclient and click on the "Invisibility_Form_Test" verb to see the bug: http://files.byondhome.com/SilkGames/bugs/ NEStalgiaWebclientBugs_v1_src.zip
Code Snippet (if applicable) to Reproduce Problem:
From the linked demo:
mob/player/var/image/invisible_form
mob/player/verb/Invisibility_Form_Test()
if(src.invisible_form)
del(src.invisible_form)
src.invisibility = 0
else
src.invisibility = 100
src.invisible_form = image('Icons.dmi',src,"InvisiblePlayer")
src << src.invisible_form