Ideally, images should have no text on them at all.
If images do have text, keep the text to a minimum (less than 20% of image). Facebook does not allow text-heavy images in promoted posts, and may not approve your post.
When possible, show places people will recognize.
If the image is a location, it should ideally be easy for the general public to recognize.
Images should be high-resolution.
Use direct questions, especially if comments are the goal. Asking a question in the post can increase participation in the comments.
Keep text brief, and use language that conveys why this would be important to the average person.
The limit on the number of characters is 63,206 characters. However, we'd recommend keeping it closer to 40-50 with an image.
Note: Facebook may reject your post if the image has more than 20% text, or if the post's text is longer than 63,206 characters.