In the image editor there's a button on the toolbar where you can specify the "export file format" where png-8 is one of the options. As far as actual memory usage by your game, it doesn't matter what file format is used because when an image is loaded it's loaded as a uncompressed 32bit image in video memory.
The best that could be done if the game designer was happy about a loss of color depth would be for c2 to use compressed textures in webgl. The issue is support of it. According to the stats here:
The most supported texture compression format is s3tc and even then it's only supported by about 75% of devices.
s3tc gives a 4:1 compression at the loss of the amount of colors.
http://en.wikipedia.org/wiki/S3_Texture ... Comparison