@Robin
Weaver’s advice is sound.
Years ago I change my database character set to handle all the odd languages and fonts I received and was using, and it worked well and stopped the error messages.
My WP-Admin browser now also handles and displays most of the Emoji etc. I need, but when I test them on various phone browsers, they just don’t display at all, or display as colorless outlined shapes. So, not useable. The caveat is: just because it works fine on your desktop pc, does not mean it works on all phone browsers. 🙂
Weaver’s suggestion may seem tedious, but if you are only doing a few, it is not too much work to ensure 100% of your readers get to see the image correctly.
FYI, I found the simplest and fastest APP to change image types is the old and dated “MS Paint” and I use it for this type of conversion daily.
Hope this helps.
Regards!