If what you are attempting to explain is complicated, add graphics: photos, drawings, tables, charts, graphs, or whatever makes the most sense. Add graphics when they add value to the work--not only to make it more visually attractive.
Your images should be of high resolution when viewed at the size the reader will see them. If it is something that might go on a large screen, test to make sure it still looks clear and appealing on a large screen. There are tools, like the one here which will measure the maximum size you could print any of the images in your documents without losing quality, should you want to.
Approximately 91% of readers read the caption of images. Captions are among the most read content on a page--only lagging behind the title (97%) and subtitle (98%).
For websites, include alternative text in your site coding that describes what the image is, and/or what you want the reader to take from the image. Sometimes what you are trying to convey in an image is able to just be written into the alternative text, but other times you may need to describe your point.
Compare the captions throughout the GRID Toolkit-- some repeat the text of the image word-for-word, but others summarize the point of the image. Consider what you might want to hear instead of the image if the image were not there.
Dyslexia Friendly Style Guide. British Dyslexia Association.
Eyetracking Web Usability. Jakob Nielsen and Kara Pernice, Nielsen Norman Group, 2009.