Instructions for Windows:
1) Find the photo you want to link and right-click. Depending on the browser, the pop-up menu will include an option to "Copy image address", or something similar. Click that.
2) In FR, type your post or reply and add this where you want the image to be: <P><img src="(paste the image address you copied here, between double quotes, by pressing CTRL+V)"><P>
So, for this image:
...I did <P><img src="http://brookline.com/wp-content/uploads/2017/05/trumpandflag.png"><P>
Press the preview button to make sure your post is what you expect.