The strange thing is that I have done that:If you have images on your Google Drive that you want to add to MR posts, then you'll need to share them, which makes them publicly accessible, then get their URL, then put that URL into the IMG tags that Weaselboy showed above.
Here's Google's info on how to share files and folders:
With the file shared, you should be able to get a URL for it. Expand the heading on that page "Share a link to the file". That URL is what you paste into the IMG tags.
I found that link by googling: google drive shared url
You can repeat the search and find more info.
Is there somewhere a guide "howto post to MR forums"?https://forums.macrumors.com/posts/27653843/
Something odd about the way Google is sharing that URL does not seem to work with inline images here. I just downloaded the image and added it to your post for you and that seems to work fine.
Just download the image to your desktop, then drag and drop it into the forum post and the forum software will handle it for you. You can either display it as an attachment, or a thumbnail or full image.
Yep, nothing about posting images.