Quake Tools

Each one of them is a single C file that should compile OOTB on any platform.
E.g. gcc -o pal2tga pal2tga.c

Their usage is pretty consistent - you supply the input files.
PAL2TGA wants a .lmp palette file, TGA2PAL wants a .tga image file - and so on.

They are all released under the MIT License. Binaries for Win32 are outdated but usable.

TrueVision Targa seems to be one of the saner image formats. The GIMP handles those fine.
Most advanced editors do...

PAL2TGA
Converts a Quake palette file into a 24bit TrueVision Targa file for direct editing.
pal2tga.c

TGA2PAL
Imports a 16x16 24bit Targa (like one generated by PAL2TGA) and converts it into a Quake palette file.
tga2pal.c

LMP2TGA
Convert Quake graphic lump files (.lmp) into a TrueVision Targa for editing.
Allows use of a custom palette.lmp file for non-standard modifications/games.
Note: colormap.lmp and palette.lmp are no typical graphic lumps - opening them
will most likely result in an error when validating the header.
lmp2tga.c

TGA2LMP
Converts a 24bit Targa into a Quake graphic lump file.
Allows use of a custom palette.lmp file for non-standard modifications/games.
tga2lmp.c

TGA2SPR
Converts 24bit Targas into a Quake sprite lump file.
Allows use of a custom palette.lmp file for non-standard modifications/games, like the others...
Arguably the most complex one, README instructions required!
tga2spr.c