First, right-click on the image you want to post and choose "Copy image address" or whatever the equivalent is for your browser.
Next, test that address by pasting it into a browser address. (Some web URL's for images don't work away from the web page they're in. So you have to test the image before assuming it'll work in a FR post.)
If that works, use the img tag like below in the FR post. Click Preview to review it before posting. Increase or decrease the width depending on how much space it should take in the post.
<img width=400 src="https://pbs.twimg.com/media/GK660bobIAAX-_-?format=jpg&name=900x900">
Thank you. I don’t remember the preview button, but I will definitely try it.