First of all, it's always helpful to state the game you're working on. That being said, it should be possible to mount the ISO. Personally I use Daemon Tools Lite for this, but there are many other programs that should be able to mount Saturn ISOs. The Saturn uses normal CDs after all.
There are many ways to go about finding graphics. The systematic approach that I'd recommend is to use an emulator with good debugging capabilities like Yabause or Mednafen. Wait until the graphics you're looking for are displayed and stop the program execution. Then dump the VRAM memory to find the images you're looking for or just look at the VDP1/VDP2 debug screen in Yabause. If you know to which VRAM address the images are loaded, you can trace back the data using memory break points (watch out for DMA transfers though, Yabause won't catch those).
Another method would be to look at the files on the ISO to see if there is something that looks like it could contain images. Then open this file in a tile editor like CrystalTile, for example, and look for image data. Be aware though that Saturn supports many different resolutions, bit depths and color modes. This means images might not be easy to recognize if your settings aren't correct. You also won't be able to find compressed images with this method.