I switched to mu4e to manage my mails at work, and also to send mails. But in our corporation we all have a signature that include our logo and some hypertext links, so I couldn’t just insert my signature and be done with that. There is a simple way to deal with this problem, I fetched the html part of my signature (which include an image in base64) and pasted it into my emacs config file this way.
(setq mu4e-compose-signature "<#part type=text/html><html><body><p>Hello ! I am the html signature which can contains anything in html !</p></body></html><#/part>" )
I pasted my signature instead of the hello world text of course, but you only have to use the part tag and you are done ! The rest of your mails will be plain text, except this part.