I saying that it doesn't matter which way you go d&d or c/p, that it is only been a few instances that either one of them worked with a particular substack, that both ways didn't work or both ways did work. But a lot more doesn't work, so I stopped doing it without wasting my time to see if I get lucky. At least for me and my pc, that has been my experience (and others also). May be different for other users and pc's.
I don't believe it is in the process of either action, both are about as basic as you can get, and probably not the fault of a seasoned user "not knowing how it's done". It's a deeper issue that may depend on the particular substack and or individual pc configurations. For instance, I have a lot of blockers and security apps on mine, running six monitors, built, configured, and code wrote by me, so I'm confident that I have the ability and knowhow to do such simple tasks as well as I'm confident that you also do.
screenshot d&d from my pc file before submit, and all pics are showing in preview. For me, the only pics showing instead of just the address after posting are the ones I d&d directly from my pc files. That has always worked, but it's more time-consuming to clip shot the pic and save on my pc, then d&d the file to post, which I have to do only from substack to here. I've had nowhere else, that is regularly a problem so far.