We all know that it's easier to learn when you get a little extra help: illustrations are the most common and at hand tool. They work around memory in a way that simple text reading cannot. This widely known fact has, unfortunately, led to overdoing by some trainers, especially in the case of e-learning.
Some illustrations and ideas were so often used that they quickly became clichés. There are some drawings or pictures that everyone who ever took an online course has seen: the pupils having a blast at the computer, two people shaking hands, business men having a meeting in an office space and so on. It's what could be described as the most common of stock photography.
Images are extremely valuable, as they can easily show what would otherwise have to be defined in too many words. It so often happens that you read a tutorial and don't understand it until you look at the adjacent print screens or photos! In order to use visual aid at its best, one must take into consideration some basic guidelines. For example, it is important to know whether the respective image is the focus of the page (this could be valid in a course of Visual communication or Arts), or the illustration of a text (like when children learn the alphabet with the help of drawings of things that begin with that letter). Either way, the images must be relevant to the course content - otherwise they just fill up space, which is noticeable and damages the "reputation" of that very course. Relevance will also make them easier to remember. It is crucial that the visual aid does not distract from the content, but rather blends with it.
Another type of visual aid is video material - more complex, as it encompasses sight and hearing. As mentioned above, looking at something makes it easier to understand than having it explained, in the same manner that so many people would rather "see the movie" than "read the book". Seeing someone cut and sew a shirt is more helpful than reading a description of the process. There is no need for fancy equipment, as this is the era of YouTube and home video, when users are more interested in the content than in the quality of the video. Again, video is extremely helpful in the case of tutorials, whether it's about how to use certain software, how to put together your IKEA table or how to bake a cake. Videos also work excellently for students: if you're teaching a poem about the circus, why not show them a video of Charlie Chaplin at the circus? Though it might be a small distraction, it is also a well deserved break. And you can make sure they take it seriously by having them answer some questions on the topic afterwards.
The third medium that can be used to further engage your users is audio. This can function on its own, but it is usually associated with text or visuals. It's not the best idea to send the same message in audio and text, like with subtitles. The same goes for course explanations, examples or other neutral-toned statements. Audio should be reserved for stories, brief explanations and such. There are people who prefer audio to text - just think of audio books - but that would lead us away from the field of e-learning.
The trick to using these media is not to overload the learner. For example, if there is already a graphic chart on the page, explaining it with text might be too much, because they both appeal the same channel: sight. Pairing the chart with audio explanation might work though, as the two come in on different channels.