Graphics are everywhere. They're on advertisements, websites, labels, magazines, and all over social media. Creating them, making them intriguing, is a big step in making your graphics.
Color is a very important thing to consider when making your graphics. Cooler colors are more likely to sooth the reader and your audience while bright, warm colors are more likely to set the viewer on edge.
For example, blue is meant to be calming and purple is supposed to invoke the feeling of wisdom, while red and orange are exciting and drag attention.
The most important thing, especially for such an image-focused industry, is to have content that the viewer can read and understand. That means using fonts like Ariel, Sans, and Times New Roman that are easy to make out and legible. Being conscious of the size of your text is important as well. If you are viewing from a distance, you want the text to be big enough to read, but not overwhelming and if the viewer is close up, you don't want the text to be screaming into their face. Using curly, fancy fonts is less likely to appeal to a general audience, because they take a minute to read fully or, in some cases, they might not be legible at all.
The message is the heart of your graphic or advertisement. When you know the message you're trying to get across, you can expose it better to people that might be interested in your book. You wouldn't be making adult ads for a book that is predominantly for children or YA and you wouldn't be making big flowery posts for a book that is for a serious adult book.
Know the genre you're advertising for to make your graphics and posts more appropriate to the book you're talking about.
The main problem with images is knowing what is too much and what isn't enough. You don't want to yell at your audience but you don't want them to be underwhelmed, so make them interesting but don't scare them with your flamboyant color swirls and big images that overlap. This can end up obscuring your text.
Contrasting colors catch the eye, so if you want to use color keep it to 2-4 that either go together very well or don't go together at all. It drags attention to what you're trying to say.
Visuals can make or break your message, so make sure that everything you're doing is coherent and easy to understand so that the viewer doesn't get confused. This leads to happy viewers and a happy you!