Use img tags with a custom width attribute. In the example you linked to, I've edited it to do just that. Check the revision history to see what I've done.
<img src="http://i.stack.imgur.com/1vIgO.png" width="375">
iPhone 4–5s: 320
iPhone 6–6s: 375
iPhone 6+–6s+: 540
If you resize an image and re-upload it as a suggested edit, I'm likely to ‘reject ...
This was broken for several months, but is now working again.
I Like grg's method, & in fact use it myself quite a lot for quick edits to posts.
There is another method, slightly more work, but this not only reduces the apparent size on screen, but also the actual image size - useful for if you're on the phone, etc.
Assuming it's your own ...
Nothing fancy going on; this is Stack Exchange's 'native' image upload support. We have our 'private' channel on Imgur; that's why there's a stack in the URL.
To actually show your image, you have to use image markdown like ![<image description for screen readers>]. This is automatically inserted when you use the Stack Exchange image uploader; from ...
Since we can't migrate this question to the main meta - please visit the main site and vote / comment on this request since it's unlikely it will get implemented for us until it gets implemented site wide.
Automatically resize high-resolution screenshots
No. The stack exchange search is for text so you would need metadata embedded in to the image in the form of a where to URL showing the image link. That text would be easily searched for.
I would recommend tin eye reverse image search or another search image meant to classify an image. Hopefully they have crawled and indexed either this site or https://...
I'm not aware of any way to edit images within the site. What you can do though:
Download image to your computer
Use the editing tool of your choice to reduze the image size
Edit the original post and replace the image with your reduced-size version
I see a difference in Urls when I select them using F12 in Chrome.
This is your chat URL http://i.stack.imgur.com/dYszY.png?s=128&g=1&g&s=32
and this is your current (at least for me visible) question URL http://i.stack.imgur.com/dYszY.png?s=32&g=1
The URL is the same, but the s = size if I'm correct and might have something to do with it.
There are many articles discussing image @2x css but one of the simplest I've seen is by WeedyGarden.net
This can be done with HTML4 and CSS2 using a media query and changing the CSS to call the high resolution image.