From Quake Wiki
.lmp (pronounced "lump") is a file extension used for several file formats in Quake.
Quake stores some 2D UI graphics in files with .lmp extensions. These image lmps contain a small header consisting of just 32bit width then height values, the image data is then densely packed in top-down left-right order. The image data itself is 8-bit paletted using the global palette. This is the same format as most of the lumps within gfx.wad, just separated into their own files.
A file with no header, simply contains 768 of palette data. See palette.lmp.
A file with no header, simply contains 16384 (or 16385) bytes of colormap data. See colormap.lmp.