For the life of me I cannot figure this out after 13 years:

I host to image hosting website. Copy the link. Paste link into my post. Click the little mountains image button. And it never works.

trying with multiple image hosting websites. Following every guide that I can find. Asking other users how and they are posting images fine.

I've never had any issues on other htmls and forums.

Do I not have image posting permissions enabled on my d2jsp account?

Judging from your recent posts it looks like you figured it out by now.

For future reference:
It's important to use the direct image URL to embed images in posts, these always end in a file extension such as .jpg, .png or .gif.
If the link you try to post does not end in a way like that it will not work.

Either copy the BBCode (Forum) links from the share link options or right click on the image itself after uploading and choose 'copy image location'.
How To Upload Your Images To D2jsp Using Imgur

TYVM. That was the solution. the direct image URL. I was copying from imagur from the wrong location. Not right clicking. Mystery Solved