This responsive slider for Zen Cart is fully integrated. Use the banner manager to upload your images and it just automatically works. The images shrink or enlarge to fit the space (so be sure to make them the full size of the space).
It's based on the Flex Slider, an ongoing project that is well maintained. The biggest problem I ran into this year with slideshows was that most of the ones I checked would not work with the most recent versions of jquery. With a Zen Cart 1.5.4 or 1.5.5 cart, the most recent jquery is pulled in, not hard-coded so many slideshows can't work with those versions.
It comes with two positions - it can show up right under the header - full width of the size (shown below) or my preference is a smaller banner that falls in that center column as shown to the left and below.