It is not a simple operation. Images have to be downloaded to a server that lets you hotlink to the web address. I usually do a Google search of images, After I find one I like , I click on the image until it is all by itself, that is, not part of a larger web page. Once you have the Web address of that image you can use HTML to post it. Img src=WebAddress in brackets.
I use the inspect element.
Sounds like a pain in the @$$. Posting to FB is easy. Why is it not here?