Is an HDPI version of stack exchange coming to our retina screens? I would be happy to see support for HDPI.
The first thing to improve is the image uploader. It should be simple to add a selector for the density of the images we upload to avoid displaying huge screen captures in our questions/answers?
See the related question: Show larger images for high DPI displays for an example of how terrible screen captures can look when taken on an HDPI screen and uploaded without prior processing.
Could you also add a way to redefine the density of current images so we can fix the ones that are currently broken without having to replace the HDPI version of our existing images?
This answer used to perfectly illustrate the problem - I edited all images with
<img> tags and hardcoded width/height values).
I believe it is better to fix this issue now, while there are few HDPI users rather than later when thousands of posts become disfigured by HDPI screen captures. Currently they really decrease readability and give a very clunky look to the site.
Also let's remember while the Apple retina thing is HDPI, HDPI is not retina. Other screen densities are already out there.
So automatically giving the option to scale to the density of the current display would be the best approach. Reveal other options only when the user requests it.
Concerning gravatar the solution should be prety simple at the moment replace all calls to http://www.gravatar.com/avatar/xxxxxxxxxxxxxxxxxxx?s=32&d=identicon&r=PG with http://www.gravatar.com/avatar/xxxxxxxxxxxxxxxxxxx?s=64&d=identicon&r=PG for *2 resolution.
Planning ahead will you add support for intermediate resolutions densities (*1.5, *1.75) and for higher ones (*3, *4...)?