I've seen theming discussion related to portlets, availability, etc. However, I couldn't find anything that talked about auto resizing of images.
I am not a themer. It doesn't appear to me that the Plone default theme is truly responsive, as I understand it. I'm trying to understand Plone theme and how it handles responsive behavior related to images. the plone.org page tells us the Plone default theme is responsive.
As a test, I put the identical image in for the logo and in the hero section
(here: http://mckennariley.info/). However, the images display different sizes; and, the top one does not resize as I thought it would because the horizontal bar dispalys on the desktop browser. finally, when viewed on a phone, it again does not resize.
Plone 5.x does auto resize images (at least their width and height, not their absolute density) and 8 out of 10 times it gets this right. Plone 5.1 will introduce support for "srcset" which is closer to "truly" responsive images. This makes it possible to specify specific images for specific screen sizes.
See the note in the Plone 5.1 upgrade guide about HiDPI images.
So the infrastructure will be there in 5.1. There will still need to be work done to get to sensible default implementations.