You can use your instagram account. Choose the photo and go to the source code.
In Chrome it's under the header View/Developer/Source Code.
In Firefox it's under Tools/Web Developer/Page Source.
Here's an example. For this photo:
https://www.instagram.com/p/B9R1bH2DIY8/
... go to the source code as described above and look for the line that says
<meta property="og:image" content=
For this photo it's on line 176. Copy the whole link starting with https.
For this photo it looks like this:
https://scontent-lga3-1.cdninstagram...29&oe=5E945DB2
Then put all of that in between the header [img] and ender [/img] so it looks like this but without the spaces:
[img]
DON'T LEAVE ANY SPACE HERE https://scontent-lga3-1.cdninstagram...29&oe=5E945DB2 DON'T LEAVE ANY SPACE HERE [/img]
If you post that without those spaces it will end up like this:
Then just below that, leave the link to the original posting of the photo so the photographer gets the credit. Like this:
https://www.instagram.com/p/B9R1bH2DIY8/
I know that seems like a lot, but it's pretty easy once you do it a couple times.