As the slideshow plugin does not yet support zp_picturefill, I've created my own slideshow plug-in. I have a JS function for substitute x1 or x2 picture but there is a conflict with picturefill.
Actually, I don't load any ZP js files (in theme-head) and load my js in theme-body.
I just wand to "unload" zp_picturefill in the slideshow.php page.
And yes, the slideshow.php plugin is enabled (for the slideshowlink function).
Sorry for the delay in response, there are lot's of photos to do these days...