Basically youtube and other videos post by just copy-pasting the URL into the DU text. Use the preview button below the text composition and experiment with trimming URLs. Check all right-click menus. There may be "embed" instructions but they are not needed. What you want to end up with and what you want to paste is a short URL (roughly something like 15-20 characters).
For photos use Imgur or some other non-throttling service.
Postimage shows the image to the poster and it goes into their browser cache. Same thing for the first lot of viewers (100? 500?), they get to see it and then it is in their cache. But the later ones are unlucky and don't see it. Then when they mention it, the OP and a few others write "Well, um, uh, I'm seeing it! It must be okay!!!" So people who post with Postimage don't know how bad it is.
As to actual posting: trim the URL back from the end to any question mark in the URL (eliminate the q mark too). It should end in a dot extension, like ".jpg". Other common extensions are .gif, .png, .webp, .avi, .mp4, and others.
In the following examples, I will insert a space in the URL to make it non-active (not detected by DU code) so you can see the URL rather than the picture.
https://w ww.boredpanda.com/blog/wp-content/uploads/2021/10/funny-ceiling-cats-315-615ab7482e818__605.jpg?code_that_%28reveals_your_browsing_and_source%2917ABadf76jluEADyufufeFAFO
trimmed:
https://w ww.boredpanda.com/blog/wp-content/uploads/2021/10/funny-ceiling-cats-315-615ab7482e818__605.jpg
proper URL without space:
URL without trimming, notice the codes at the end of the picture
?code_that_%28reveals_your_browsing_and_source%2917ABadf76jluEADyufufeFAFO
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