If you want to display a date when the editor has saved new conentent you can use the Saved Date instead. By not automatically update the Changed Date the editor can make minor changes without the date getting updated.

Yeah but that does not force the browser to refresh content, only Modified date will.

This has nothing (well not completely true) to do with the modified date sent in the http headers. You can control these headers by yourself, one way is to replace the static file handler and rely on etags instead. But I would instead generate unique names for every file version (we do this for every image in our solutions) so you can have a far future expire date, like a year or so. So when an editor updates an image it gets a new url in the templates.

You can also disable client cache or set it really low. Which has nothing to do with Episerver really, that's more an IIS setting.

Still why would you need to "replace the static file handler and rely on etags instead" when EPiServer should just leave the checkbox "Update modfied date" checked by default (which is my preferred solution through a published event). No solution is more simple than that.

Newbie EPiServer developers shouldn't need to worry about this ... imo bad design decision, and I still haven't  seen any good argument why modified date isn't updated by default. I know someone might want a modified date manually updated in specific scenarioes for frontend purposes ... but one could always add a date property for that.

And on a side note ... it is fine that you use unique names for each file version, but then the image will not get updated automatically where ever it is used. In xhtml or url properties etc.

I can agree with you on MediaData, that the modified date should automatically update when it's just saved. But for pages I think it works great as it does now. Otherwise saved and modified date would always be the same.

The unique name is of course generated when the page is rendered. Internally a permanent link is used that never changes. This is the most fool-proof solution and always works great with CDN and other proxies.