I'm sure I am not the only one who has searched the Internet for the correct dimensions for the 2015 Twitter cover image and I could easily find that the recommended size is:
1500px (width) x 421 px (height)
BUT, Twitter's set up is complicated and although I would set up my image to the correct dimensions it was cropping it and I was losing image quality.
UNTIL, I found this free downloadable template.
I am grateful that I found this template. IT WORKS! This template has been tested in different screen resolutions and in Twitter app using iPhone (5).
TIP: Be your file to the highest quality and be mindful of the "visible" and "invisible" areas on the template.
If you're looking for a custom Twitter design, no problem! I can help!
Contact me jeni @ thehavencreative.com