If using the Twitter for iOS app, it is possible to copy and paste images, emojis, and GIFs to Tweets and Direct Messages from 3rd party keyboard apps.
To add an image, emoji, or GIF on iOS:
When composing your Tweet or Direct Message, tap on the text field to bring up your iOS keyboard.
Tap and hold the globe icon on your device and select the 3rd party keyboard you wish to access.
Tap and press the image, emoji, or GIF you’d like to use. When prompted that the image has been copied, tap and hold on the text field.
Tap Paste to add the image.