That said a picture is worth more than a thousand words. In this quick guide, we will show you how to create an eye-catching Twitter cover photo using the free image editor, Photoshop Express, on iPhone and iPad.
Add image(s) to the editor
Your header photo should reflect your personal or business brand. For example, you can use your logo, product pictures as the banner image in Twitter. The header photo at the top of your Twitter profile should be at least 1500 x 500 pixels. It will look grainy if the picture size is too small. In this case, you may consider joining together two or more pictures to create a collage photo as the Twitter cover photo.
Run Photoshop Express on your iPhone or iPad. It will open your Camera Roll so you can import photos from it to the editor app.

Switch to the Collage tab on its home screen, if you like to create a collage photo as your Twitter cover photo or banner image.
Create Twitter header image from photos on iPhone
Once you have added the photo(s) from iPhone Camera Roll to the image editor, tap on the Crop button at the bottom, then choose Twitter Header beneath the Aspect Ratio tab.

Touch the Share button at the top right corner to open the Save & Share screen.

Preview your Twitter banner image, touch the Size to change it to 1500*500 if you like to set it to the minimum size. Optionally change the JPEG quality. Finally touch the Camera Roll button to save it to your iPhone. And you can upload it to your Twitter account from there.
Create collage photo as Twitter header
Photoshop Express can help you easily make collage photos based on templates. Under the Collage mode, you can select a template or layout, then save it as Twitter header. You can find more details from this guide to combine photos on iPhone.