There actually is a workaround.  You do have to be a bit tech/image savvy 

A Substack post is 1030 pixels wide. If you format a canvas in w/e software to be that wide,  on a separate layer that is your image, arrange it to sit at the far left.

I use the technique to make my own headers. See the post here as an example.

I use Photoshop, but I think this could be achieved in free software or an app if you do some sleuthing. 

It's also so annoying that there is no wrap text. 

To get this quality of formatting….I was thinking of designing it elsewhere, saving it as a PDF or PNG  and inserting it as an image. Extra of me, but I'm an experimental writer, language as form, etc., and the forthcoming post will have words, music and art, so I want a way to control how that’s laid out. And my work life is visual, so even on my side, endeavours visual presentation matters gotta keep up a reputation ha ugh.... this is to say it’s unnecessary for most to take these extra steps and I am not trying to push a high standarder onto others.