http://tipjar.alexandra-douglass.com/
https://twitter.com/MSPAArtTeam
Partial archive of the Homestuck Art Team Tip Jar, which ran from February through August 2011. Wallpapers rotated each month with a donation.

February:
"Terezi: Investigate" (2800 x 1575) - By Lexxy
SUPER SECRET BONUS WALLPAPER - By Eyes5*

March:
"Land of Pulse and Haze" (1920 x 1080) - By Sockpuppy
"Katana vs Chainsaw" (2133 x 1600) - By Paige Turner
SUPER SECRET BONUS WALLPAPER - ??????*

April:
"Trollcestors" (1920 x 1080) - By Nic Carey (Tynic)
"Frog Catching" (2300 x 1275) - By Vivus

May:
"C.R.U.N.K." - By M. Thomas Harding (Supercar)
"Magic vs Science" - By Richard Gung (Ink)*

June:
"Strict Beats" - By FauxMonstur (Katie)
"The Dame is Bad News" - By Brett Muller (Odyssean)
BONUS WALLPAPER - By Lexxy* [Probably 'grimdark']

July:
"Can Town" - By SaffronScarf
"LOWAS" - By SkepticArcher

August:
"White Host, Green Room" - By myluckyseven
"Sprites" - By clorinspats*

*Not found or unknown. It's highly likely that the unknown bonus wallpapers were amongst those that were sold as prints on the WP store in late 2011 onwards.

Also if you're wondering about julypaper.zip, so am I. It was in an old .rar I had of a few art team wallpapers that claimed all of them were from the tip jar (some of them were in fact not from the tip jar). The name, multiple size bundling and high quality would indicate it's likely from the tip jar except that there was not an unknown bonus wallpaper for July so *shrug*.

'Grimdark Rose' was a doodle during March, complete no later than June 13 2011, and never posted fullsize publicly by Lexxy (but sold as a print on the WP store), making it highly likely to have been June's secret wallpaper. It has a 'Thanks!' written next to Lexxy's signature, which was also on the 'Terezi: Investigate' wallpaper.

Included the thumbs of 'Magic vs Science' and 'Sprite'... just so you know what they looked like.