Finding a .gif
To find a .gif you want to use, your can do two things. First, there’s a search field, located in the top right corner of the blog, next to a little magnifying glass. The better way to search, though, is by tag! Go straight to the URL; it gets you much better results. Use the “gif-database.tumblr.com/tagged/put+your+tag+here” format.
Using a .gif
If you want to reply to a Tumblr post with a .gif, the best way is to click on the “Insert/edit image” button (the little tree icon above the text box). Paste the URL of the gif, and now here’s the important part, delete the numbers and the period between the http:// and media. Press +Insert, and you’re done! That ensures that your .gif will show up on people’s dashboards rather than being a little grey box that people have to click on.
The other thing you can do to make sure it’s doesn’t turn into a grey box is to upload it from your computer! That means rather than insert it into the post using the URL, save the .gif to your computer and click the +Upload Photo link above the reply box.