you could convert image to base64 url encoded... but that could be heavy in source code:
https://code.sololearn.com/WljGVfpz1Dp1/?ref=app
there are tons of online free converter that you can easily find by a google search ;)
absolute urls could skip protocol (it will be inherited by the page url protocol) by only using //:
//www.example.com/directory/file.ext
however, this means that your image host should handle both http and https protocols, as you don't know wich one will be used ^^
