Here's my first pack of loading screens made from Blizzard artwork. Mainly screens that were at first meant to be in the game but were scrapped, and some others from released artwork.
PACK 1: Braxis, Char, Kaldir (HoTS), Landing Dropship (HoTS), Mar Sara (scrapped version), Moria, Protoss ship w/ Kaldir at the background (HoTS), Shiloh, Umoja.
SC1 + BW PACK: All loading screens from Starcraft and Brood War in 1024x768 resolution, so they are not widescreen friendly.
PACK 2: Avernus (Beta version), Char (From the invasion cutscenes I guess), Earth's Supercarriers (Blizz concept art), Hyperion (WoL development image), Mengsk staring at Tarsonis (From WoL cinematic), Overmind killed (WoL concept art), Planet Cracker (WoL concept art), Swarm invading Korhal (HoTS wallpaper), Thor vs Vulture (Blizz artwork).
If you know Blizzard artwork that could be made into interesting loading screens let me know and I'll work on them.