The game's textures, sprites, icons are all in a custom (headerless?) .tiff format, as indicated by character detail files, perk/aspiration functionality/appearance files and prop behavior files.
Normally, they'd actually have a .tiff extension, but due to the filenames being obfuscated in the main archive (pathcache.bin) they're all in .RES. Luckily, all .res files have a standard header which details what kind of data file it is (script, music (Wave format), sound fx (ATRAC3+ format), texture, etc.)
Uploading .res files is not allowed, and renaming it to a .tif didn't work, so I uploaded it at https://mega.nz/#!bpIHHT6B!pQ5fmWLufkPK ... jfHIY3hCcI. It's an example file of a texture (txt_cleaningtoiletbrush_bristles.tif). Actual data for the file is between 69 6E 64 78 at the beginning and 73 75 72 66 at the end.