In the screen that follows, the last option on the right is a new GIF. To insert a GIF, tap the little plus icon in the lower-right corner when you're composing a message.
The tweaks are only for Yahoo's Mail app, not for the browser-based version of Yahoo Mail, and they're pretty simple to use. The updates are designed to ease the process of attaching Google Drive and Dropbox files, as well as GIFs from Tumblr, to outgoing emails (for all those times you just need to send someone an animation of a cat eating a slice of cake). Still using Yahoo Mail? The company might be on the auction block, but it's still working to improve its various core properties, and it has announced a few new tweaks to its Yahoo Mail app.