Image compression?
CubeCart takes a source image, typically PNG or JPG, 750x750 pixels for example, then creates sized variants specific to the intended layout area and page. The source is uploaded to /images/source/, and CubeCart's variants are saved in /images/cache/.
The specific variants are coded in the skin's config.xml file. Take a look a Foundation's config.xml file in a programmer's text editor.
Here, you will find image nodes with the reference attribute indicating where the image is used. The product page uses 'medium' and 'gallery'. The maximum attribute specifies the length of the longest side, maintaining proportionality, but never longer than the original. (That is, if the source is 450x450 pixels, variants could be smaller, but will never be larger than that.) And, of course, the browser can scale the image to fit the layout's placement if the HTML coding allows it.
Note also the quality attribute. For many image formats, there is the 'compression ratio', expressed as a 'quality' -- 100 being very lightly compressed, if at all, and 10 looking like crap.
I have visited web pages that initially show a super-blurry image (high compression), then eventually, under javascript control, fetches a high-quality (large filesize) image to replace the blurry one.
Currently, CubeCart does not do this.