@Janet photoshop, any version really.

the general process:

- load the photo, resize it to be small (i usually get best results between 250-600 pixels on the long side)
- file > export > save for web (you can resize it here too, make sure to use "nearest neighbor" scaling)
- change file type to .gif
- the main things to change in this window are color count & algorithm, and the dithering style.

this one was 93 colors, "windows" color algorithm, diffusion dithering style.