You really should save these as PNG-8 instead of jpeg. They should compress smaller, and they won't get compression artifacts because PNG is a lossless format.

Jpeg is most appropriate for photographic images with lots of color gradations. PNG is an improvement on Gif, and it does best with line drawings that have large areas of solid colors and repeating patterns. And you can get even more savings by limiting PNG to a 256 color palette (PNG- instead of the full 24 bits per pixel of a jpeg.