I don't know if there's any pictures, but in the "History of Lemmings" thing that one of the DMA people (I want to say Mike?) posted a few years back, they mentioned the editor for L1 was built into the game itself and very Deluxe Paint-like (whatever that is).
Some of the 8-bit console ports (NES, Master System, etc), the levels are simply tilemap-based - this shows in the gameplay on NES and Gameboy, but Master System version uses some ingenious hacks to get around it being too noticable. These were in all likelihood made with a general tile-mapping tool.