I tried fixing this “issue” in the past but got busy with other things. I had actually mentioned the solution in my original post without knowing it, but it slipped through the cracks I guess and was forgotten about until now.
The file /etc/chromium.d/dietpi contains a set of flags that the DietPi devs thought would be useful (they are). One of them being --enable-low-end-device-mode. This will force Chrome to use 16bit color instead of 24, among other things. Removing that flag improved the quality of images displayed in Chrome. I haven’t noticed any performance issues yet, but we use our Pis to display static web pages, so your mileage may vary.
See below:
