toaru_jpeg is a standalone version of the ToaruOS JPEG decoder, which is itself a rewrite of Raul Aguaviva's Python "micro JPEG visualizer" in C.
This is a minimal, incomplete implementation of the JPEG standard - enough for basic images, but not for any advanced features. Mostly that means non-progressive, 24-bit RGB color images only.
This tool produces a TGA image from the input JPG, so you can use it like this to verify it is working for your image:
make
./toaru_jpeg ~/path/to/some/simple.jpg
eog out.tga
The Toaru JPEG decoder is itself released under the terms of the NCSA / University of Illinois License.
Raul Aguaviva's Python "micro JPEG visualizer" carried an MIT license.