Yeah, so I tested out the screenshots... and the high res, lossless format still works fine. You just have to put -bmp in the command line for the shortcut to GW2.
(Note that I convert them to png before I put them on an image host. bitmap files are friggin' huge.)
Here's my Necromancer at 70. Eventually, I'll find some gloves and shoes I like that go with her growing-out-of-me plant armor.