The way I do it is by finding a GIF via an image search, and then right-clicking on the GIF and selecting "copy image URL". That will vary somewhat depending on your browser, like Firefox says "copy image location", if I'm not mistaken.
Then, when you're making a post, click the little button (above the area where you're typing) that looks like a framed image of a tree. In the box that pops up, select the tab that says "from URL", and paste the URL in the appropriate space. UN-check the box that says "retrieve remote file and reference locally".
Tryyyyyy it! Find a fun GIF! I'll wait!