Jarallax not working with <picture> in Firefox

[chesio]

Issue description:

I have an <img> tag embedded in <picture> element with additional <source>-s for different screen resolutions. In Chrome, the correct <source> is picked up by Jarallax, but in Firefox, the additional <source>-s are ignored.

Version used:

• Jarallax 1.10.3 installed as Node module
• Firefox 59.0.2 on Windows

Code to reproduce the issue (HTML blocks + JavaScript initialization)

Please, see the demo page at: https://www.bluechip-devel.at/jarallax/

Note: To better illustrate the problem, there are two identical <picture>-s on the page. The first one is Jarallaxed, the second one not. On large screen (1024px wide or more) in Firefox, the first image is blurry, while the second one correctly loads larger version of image as provided by <source media="(min-width: 1024px)"> tag.

[chesio]

I did some more digging and found out that the jarallax-img class has to be set on <picture> tag, not on <img> tag. I've updated the demo accordingly, now it works in Firefox as well :-)

Maybe it would be worth to add a related example to README?