Not every image has that exact url form. But this trick still works! And it kicks the ass off having to go do it elsewhere before posting. I dropped an image in the text editor and it came up as "https://steemitimages.com/..." followed by a lot of other random crap and then the name of the file somewhere in there.

I simply added this before it "https://img1.steemit.com/300x800/" so that I now had "https://img1.steemit.com/300x800/https://steemitimages.com/..." and it worked!

Thanks for the awesome trick!