How I do gifs on #mastodon when using the desktop browser.
I installed the Giphy extension for Chrome (It works on Microsoft Edge, also. Most Google Chrome store extensions install on Edge)
I then click it in my toolbar, search for a gif and then paste the link since Mastodon shows them. You don't need to download the gifs and upload like an image.
https://chrome.google.com/webstore/detail/giphy-for-chrome/jlleokkdhkflpmghiioglgmnminbekdi

