Regarding Sega CD assets, it usually uses the same regular Megadrive graphical formats, so Crystaltile2 on tile mode N64/MD 4BPP should be alright for most ripping and viewing (as long as it uses the proper palette so that colors show correctly... I usually recreate 16 colors by hand but probably should write a converter one of these days).
Compression was a concern for cartridge games but the polar opposite of those practices was in effect for CD based games. Cartridge games would leave anything frequently used (fonts, HUD, player sprites) uncompressed while compressing less frequently used graphics (cutscenes, etc..). Of course exceptions always existed.
The many ways I used to find graphics, but by no means an exhaustive list:
- Uncompressed: much more frequent than what you'd expect.
- Nemesis: graphical compression used for a LOT of first party Sega games, and even third party ones. There's a command line tool that detects and dumps everything using it, and another one (that covers three less frequent compression formats used by Sega) that can compress and decompress as long as you have the offset.
- Run length encoding: it's often obvious enough even in compressed form when viewed in a tile editor
- Rob North Compression: Graphical. Used by European developers and in Sonic 3D. (idk if there's a tool for it here)
- Toshio LZ: Used in Crusader of Centy.
- Konami LZ: 3 revisions. Graphics mainly. A command line tool by Lab 113 is on RHDN's database, as well as another one for the SNES variants.
- Westone LZ: two variants, but one was used a lot. Used in MW3 and MW4.
- A compression type Game Arts used for Lunar and a bunch of other games.
I recommend ripping them off the VPU (VRAM) memory and passing them to a tile editor, using any debugger-capable emulator that allows so. The game needs them uncompressed after all to make use of them at some point.