My main work on Wikimedia Commons was to optimise images of maps (in terms of file size, color diversity and compression format). I optimized several dozens of maps.
The largest part of them are related to my country (Bosnia and Herzegovina), but I also optimized some maps of ex. Yugoslavia, the Balkan region, and even some wider-regional maps (like some maps of Roman, Austro-Hungarian, and Ottoman empires).
In many cases, the file size reducion was substantial, even enormous, while the image appearance remained the same, or EVEN BETTER in many cases!
The images whose size I managed to reduce, along with improving their appearance, are those which featured dihtered colors. Dihtering is a VERY "troublesome" technique when not used on the right place with the right purpose. One of the main problems which it brings is that it makes the image much more complicated for the compression algorithms (like the ones in GIF and PNG) to process, resulting in a very low compression efficiency. By replacing the dihtered areas with matching solid colors (not an easy nor quick job to do!), I have optimised both image appearance and the file size. Isn't that marvelous!?
However, I have not been active on Wikimedia Commons for the last year or so, and I do not plan to do much work here in the near future (because of the lack of spare time). Besides that, I have "exhausted" the mapping material related to my country.