Action Details
Requirements and how-to articles for Twitter's action:

paid action
A website card image is used in a promoted tweet to encourage users to visit your website.

You need this to get it done:

Website Title
The title of the website you are sending your audience to (maximum of 70 characters, recommended no more than 50 to prevent truncation.)
Tweet Text
Text to accompany the website card and link (maximum of 280 characters.)
File format: Recommended .jpg/.png (.bmp/.tiff not supported)
Maximum file size: 3MB
Aspect ratio: 1.91:1 or 1:1 (1x1 aspect ratio not currently available to self serve advertisers)
Image size: 800 x 418 pixels (for 1.91:1 aspect ratio) 800 x 800 pixels (for 1:1 aspect ratio)
The URL you would like to send your audience to.
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Twitter Business - Advertiser creative specifications
Official information on creative specs for ads.
Link missing here or found errors? Improve How-Tos