If someone is trying to post a picture, and seeing the [img] tag show instead of the picture, it's one of two things.
1. "Disable BBCode in this post" has been checked. (Either because of accidentally clicking it, or because "Always allow BBCode" was set to know in one's profile".) (This doesn't seem to be your issue.)
2. The URL is not a valid photo URL. (This is the issue.)
Flickr does have URL's that end in .jpg or .gif
BUT, don't use the URL in "Grab the link". That gives the URL to link to the photo page. It's what you'd use to link to a photo but not have it visible in your post. It's the same as what shows in the address bar.
You have to use "Grab the HTML/BBCode". Select "BBCode", select the size you wants, copy the text in the box, and paste it in your post. It already has the IMG tag, so no need to add that. If you don't want that text under it, edit the BBCode it gives you, though Flickr does say we are supposed to link back to Flickr when posting pictures elsewhere.
I just used that yesterday on another messageboard to post some pictures. So, yes, I've used this.
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not aspie, not NT, somewhere in between
Aspie Quiz: 110 Aspie, 103 Neurotypical.
Used to be more autistic than I am now.