With Lemmix players (at least if they're my updated versions, which add the mass dump features), you'll want to use F5 for level dump and F6 for image dump, IIRC.
I can probably put together some kind of command-line app that does this for you if you like. Do be aware it'll be based on NeoLemmix code, so (a) special graphics levels won't be supported, as NeoLemmix doesn't support that anymore (instead, NL "special graphics" levels are created by using a custom tileset with a very large terrain piece, and handling objects by taking advantage of NL's ability to mix multiple tilesets in one level), and (b) there may be some very minor differences, such as that NeoLemmix has differently-colored one way arrows for the Snow graphic set than DOS L1 / Lemmix do.