Here is an image converter app (.TGA files only) that will convert the images to both c source code and binary file in a 4 bits per pixel format. The app is super simple to use : put it in your image folder, launch it and just type de filename without the extension. It will produce 2 files.
I've also included a demo to use it both for 16 Colors Lookup Table sprites (VDP1) and 16 Colors Bank (VDP2 Color RAM). The demo loads one sprite from c code and two from binary files. It displays one sprite using CRAM and two using CLUT. It's using JO Engine in a ghetto fashion as 4 bpp isn't supported, so let me know if it creates issues with your apps.
Using 4 bits per pixel sprites helps reducing the VRAM usage while also easily allowing you to use advanced VDP2 effects.
I did it quickly and didn't do much testing, let me know if it doesn't work well!
EDIT : Added a 4 bytes buffer to allow space for the data pointer