You can try using an online email signature generator such as Gimmio . Modify one of their templates and copy to your clipboard. Go to Settings > Mail > Signature and delete the existing signature. Paste the copied signature.
Modified template and HTML source code:
Copied to Settings and used in a compose email:

You can copy the HTML source code and modify it in a code editor such as Textastic. Here is the existing 30 px text:

And the modified 15 px text:

Save any changes made in Textastic to iCloud Drive in the Files app. Copy the modified HTML document to paste in to Settings. You can also change the existing image hosting site, StackPath in the case of Gimmio, to one of your choice using a direct image URL.
I haven't tested over time for persistence, so test as needed.
iPad Pro (12.9-inch) (3rd generation), iPadOS 13.5.