lulz there is no such thing as compressing a image you can only optimize it to make the file size smalller. Putting it in a rar or anything of the like does nothing it just makes all the pretty images stay together really no difference with zip or unzip unless that is if you plan to distro said images than rar zip or w/e is useful. image editing software like adobe helps with these thing to mall files smaller.
There Are differences in the internal compression algorithms, depending on chosen image format. In so far as optimization, it's generally the removal of redundant data and/or psychovisual based tuning to increase/maintain perceived image quality to the human eye, whilst simultaneously decreasing size.
And as previously stated, using rars/zips/7zs/etc are generally useless for additional compression. Even under the most optimal conditions and utilizing the highest available compression settings for the aforementioned archivers, you would only see roughly ~0-5% improvement.
However, in addition to following the suggestions from Sangaz in regards to lowering the bit-depth/grayscaling images via PNG-8; you can also run these files through a lossless image optimizer to further shave the size downwards in "Some" cases. A great site for doing this, is Yahoo's "SmushIt" @ www.smushit.com
If you wanted to go to the next level, you could utilize something like IrfanView that supports RIOT/PNGout; its recursive analysis & tuning passes allow it to reduce the size even further.
The JPG Image that Sangaz provided @ 287KB (100%).
Standard PNG-8 Conversion / SmushIt @ 95KB (33.10%)
The IrfanView/RIOT/PNGout Xtreme optimized PNG-8 @ 88KB (30.66%).