Email signature with image on iPhone
The original thread, below, does offer a potential solution, but it misses one important point (at least for me on my iPhone 12 running iOS 15.2,1 it's important), in that it only works with text AND an image if you enable All Accounts, which means one email signature across all accounts. Multiple text-only signatures work just fine, but it seems you can't have one signature with text and an image and the others text only. It still strips out the image.
The vast majority of people have one email address on their iPhone using Apple Mail, so using a common signature isn't an issue, but I have four accounts for different projects, so having to use the 'All Accounts' option is a royal pain. It's good that it works on the main account though.
Original thread:
e-mail signature on iPhone with image - Apple Community
To be honest, I may just go back to the Spark app as it's nothing short of brilliant and has none of Apples signature issues and faffing around.
iPhone 12, iOS 15