Icons, Clipart and Photos
There's a fine line between having no pictorial elements and having too many. Photos, icons, shapes and clipart are all excellent ways to add something interesting and enjoyable to an otherwise solid block of text. The human brain likes pictures, and it helps solidify the information in the mind of the reader. As an example, let's look at the below Powerpoint slides.
Both slides hold identical information, but the second slide looks considerably better and more memorable. Why? Because of all the aspects we have described in the previous pages:
If I had added more than three pictorial elements to the slide, that would have been overwhelming and unnecessary. If I had used only one icon, it would have been okay, but it would have needed to be sized and aligned in a way that the reader would easily understand. There's a balance between cluttered and empty.
If your poster was an essay, think of the pictorial elements like punctuation: they increase understanding and clarity. No punctuation makes it difficult to pause and remember. Overuse punctuation and it's clunky and confusing.
NOTE: When searching the internet for icons or photos make sure you aren't using someone else's intellectual property. You can always purchase the image or icon from the host, but if you would prefer some free alternatives use Google's image search feature and under "Tools" you will find "Usage Rights" and filter by "labeled for reuse".
Other great free to use online resources:
The Noun Project (icons): https://thenounproject.com/
Unsplash (stock photos): https://unsplash.com/
Ohio State University also has a list of public domain and open resources: https://library.osu.edu/copyright/public-domain
...And there is a wealth of other resources online here (with some copyright explanations) : https://www.salehoo.com/blog/free-stock-image-and-photo-resources