Good question! Design is not just images. It is the entire visual display. It is the presentation. It is the lifeline of your landing page.
A landing page needs to be clear and concise. The text needs to be simple. If you are using complex words, more often than not you will lose your audience.
The design of the landing page also needs to factor in margins and empty space. The point of the empty space, specifically, is easily overlooked. In a world where we can have access to anything and everything and at any given moment, it is necessary to provide this empty space, because in doing so we bring out the key elements. If your landing page is filled to the brim, then you are probably losing traffic due to the fact that your call to action is not standing out the way it should be.