Unlike on Twitter where you can choose to include an image in your tweets, adding an image is required for you Facebook post/ad. The recommended size of your images should be 1200 by 627 px.
Facebook lets you choose up to 6 images to show in your ad, this will rotated as the ad is delivered to your audience, eventually favouring the ones that are performing the best by displaying them more often.
One important restriction to note is that your image can only contain 20% text.
Facebook gives you plenty of space to make an impression with text. There is a headline title (25 characters), text above the image (90 characters), a call to action button, and a news feed link description (200 characters). It is not vital that you fill out all of those areas, just write as much as needs to be communicated, and pick your words wisely.
It’s key to ensure that you address a pain point in your copy. Why should the user click on your ad? What values and benefits are you and your company providing? Don’t forget to include some kind of call to action, like ‘download’ or ‘learn more’ to compel the user to do what you want them to do!