Links to Google Drive folders that collect various Idolm@ster sprites and other image resources that I haven't seen archived anywhere else. The Cinderella Girls stuff comes from the Cinderella Girls mobage.
CINDERELLA GIRLS
Adorable official phone wallpapers illustrated by Kumajet and released monthly since 2016. Updated with a new wallpaper every month. UPDATE 12/25/2019: Unfortunately, this feature has been discontinued and there will no more new calendars for 2020 onward. However, feel free to enjoy the old illustrations!
A set of 48 official Japanese hanafuda playing cards featuring the cast of Cinderella Girls.
Character portraits from the Dice DE Cinderella fantasy RPG minigame. Currently there are over 80 characters on the roster. A new character is added to Dice DE Cinderella every month.
SD sprites from the Talk Battle Show events in the Cinderella Girls Mobage. They come in sets of four. Updated every Talk Battle Show event.
Sprites from Idol Challenge events in the Cinderella Girls Mobage. They come in sets of two -- one transparent card sprite and one smiling alternate! Updated every Idol Challenge.
Sprite sets from Idol Produce events in the Cinderella Girls Mobage. They come in sets of five and feature many outfits and expressions not seen in Starlight Stage. Updated every Idol Produce event.
SHINY COLORS
Chibi sprites for all Produce cards in Shiny Colors. Includes both casual and Fes outfits. Also includes chibi sprites for all shop-exclusive outfits. Updated with every new Produce card or shop update.
Full-body album sprites for all Produce cards in Shiny Colors. Includes both casual and Fes outfits. Also includes full-body album sprites for all shop-exclusive outfits. Updated with every new Produce card or shop update.
NOTE: These are only for showing a preview of each outfit in the album so they don't have multiple expressions. In some sprites parts of the character or their outfit get cut off slightly.