It is purely down to the person who is responsible for putting the package on the repository, and in some cases how the repository itself is set up. They can choose how much additional information to provide including description, screenshots, change log, etc. etc.
It's always best to scroll down past the description and check the sub-section links. The vast majority have a screenshot hidden somewhere, but usually in the "Screenshots" page.
Also bear in mind that some tweaks do things which can't be shown on a screenshot.