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The app has likely saved your login credentials but if you will need to Log In.  If you do not have the Twitter for Android app on your Android, you will need to download it from Google Play. If you do not have an account you will have to Sign up. You have 140 characters to let the world know what’s happening and the GIF you are about to post will make your tweet that much more engaging.  Tap on the feather pen icon to open the text box. Tap on What’s happening? and enter your tweet in the text box. A GIF is a digital image, static or animated in a standard format that your followers will be able to view from whatever device they access Twitter.  To attach a GIF that you have saved on your phone, tap the camera button and select the GIF to attach. To attach a GIF from a library of available GIFs, tap the GIF Icon to search for and select your GIF. A world of followers is waiting to click, favorite and retweet. Tap Tweet to post.

Summary:
Open the Twitter for Android app on your Android Device. Enter your Tweet. Attach your GIF. Post your Tweet.