How to put a GIF in an answer on tumblr?

if someone asks you something in your ask box and you want to make your answer a GIF, in the answer you click HTML and you're supposed to insert the GIF's HTML. the only problem? how am i supposed to know what the HTML is if the GIF is saved to my computer already? it's under "my documents" in a designated folder. how do i put one in my answer if it's saved to my computer?