As usual, when the update goes live, the default background will be the new hero, San Feng, but if you’d like to select one of the others, just go to the Preferences --> Interface tab on your profile and pick the one you want from the dropdown! (You may need to refresh your browser to see the new choices due to CDN and browser caching, btw.)
NEW FOR THIS UPDATE: Because 3.10 was such a short patch, the backgrounds from 3.10 will be sticking around through the entirety of 4.0. Also, these backgrounds are live right now, so go ahead and give them a try!
Thanks, @hazeleyes! For my mobile device, San Feng is too large and a lot of him is cut out by the screen. I don’t seem to see that issue (or it’s not as bad) with other backgrounds. Figured I’d let you know in case that is unintended. If it’s unavoidable, then apologies for the post. I imagine they’re all great on desktop.
Unfortunately, it’s basically due to the layout of the actual splash art. He’s way to the side instead of being in the middle like most of the subjects have been.