Twitter enables the addition of photos in two places: your tweets and your profile. Click the Add Photo icon on your Home timeline to add photos to your tweets. Visit your Profile page to add a new profile or header photo.
Add Photos to Tweets
Sign in to your account using the Twitter homepage and click the Home tab.
Click your mouse in the What's Happening text box and enter your tweet.
Click the Add Photo icon located below the What's Happening box.
Select the photo you want to tweet.
Click the Tweet button to tweet your message.
Add Photos to Your Profile
Click your profile picture in the top corner of any Twitter page and select View Profile on the drop-down menu.
Click the Edit Profile button on your Profile page.
Click either Change Your Profile Photo or Change Your Header Photo depending on which image you want to change.
Select Upload Photo and choose your new photo.
Click the Save Changes button when you're finished.
Posting an image varies slightly between Android and iOS devices.
On Android phones and tablets, tap "What’s Happening" and then touch the "Camera" icon to take a new photo or choose an existing one on your device.
On iOS devices, touch the "Tweet" icon and then tap the "Camera" icon to take a new photo or select an existing one from the library of photos on the device.
Twitter accepts photos up to 5MB in size and animated GIFs up to 3MB.
Accepted photo formats include JPEG, GIF and PNG.