Nice looking work. Compared to what currently ships with phpBB by default, these look like the cleaner and more high-DPI-aware images one might expect to match the improvements phpBB already ships with in prosilver for the forum icons and FontAwesome usage.
On cheesy smile (

) I'm not sure the size of your mouth truly grasps the cheesiness of the situation.

My opinion, the mouth is oversize for a reason.
Any intention to address animation? Don't know if that would be considered "important" for how phpBB ships or not. At least one animated replacement proposal had been preempted in the past by pointing out that APNG support isn't widespread among browsers, and
the APNG penetration still doesn't look much greener now. But if this just meant non-APNG browsers simply saw "a high quality but stationary image", my own personal opinion is "that's still worth the quality improvement." SVG would let us animate, but phpBB Smilies doesn't support SVG yet.
Just as a background note, I performed a test yesterday because I wasn't clear on whether providing an "oversize" Smilies image (e.g. 64x64 or 128x128) would actually be used in a way that improves the display on high-DPI screens.
Meaning clearly, if we were asking for the 128x128 image to display at 128x128, it would look great that way.
For the normal case in phpBB Smilies though, we're asking for that 128x128 image to be displayed at only 16x16 or similar. And so we get whatever downscaling quality the browser is willing to provide for rendering this otherwise high-quality image at only 16x16.
But when "display this 128x128 image at only 16x16" is performed on a high-DPI display, I wasn't sure whether the additional available source image resolution would actually be used. Versus whether it would still be a case of "the requested 16x16 display being stretched to fill the higher DPI screen space", similar to when the actual source image is 16x16.
At least in my testing on Chrome, it appeared that this works exactly how one would hope it works. Meaning if you give a 128x128 image, set it to display at 16x16, but then zoom or move to a high DPI display, you can end up seeing more of your original 128x128 image and not "a stretched version of the 16x16 that was requested."
I'm sure all that was obvious to someone else; it just wasn't something I knew or had knowingly observed before.