GIFs are used on Tumblr to show reactions or replies to other people's posts. To add a GIF to a reblog, you should have the URL of the GIF. The post will be added to your Tumblr blog and when other people reblog your post, the GIF you added will be included on the reblog.
Hosting GIFS Online
Before you add a GIF to Tumblr, upload it to a photo sharing site so that the GIF is online. Sites like Imgur, Photobucket and Free Photo Hosting offer a free place where you can upload your GIFs. Once the GIF is on the site, a direct link to the GIF will be available that you can copy and paste into the appropriate place on Tumblr.
Click the "Reblog" option to open the post editor where you can add edits or comments to the original post. Click the camera icon and then insert the URL of the GIF you want to use to respond to the post. Click "Insert" to add the GIF to the reblog. Click "Reblog post" to upload the reblog to your Tumblr page.