Those, he says, are the key factors to deciding whether an image is worth including or not. Now, as you might guess, the blog post is about slides, but the principle is a good one for poster presentations, too.
As an example, let’s examine a few graphics you often see on posters.
Logos bookending the title. They are of questionable relevance. They are often not unique, in that you may be a conference where many of your colleagues are presenting. And a lot of those university logos look pretty much alike (coats of arms, etc.).
Funding agency logos: Low relevance and even less likely to be unique.
Data graphs. Highly relevant, but often shaky on the uniqueness criteria. One bar graph looks much like another. In a way, this can be a strength rather than a weakness, since people can understand standard graphs better than unusual graphs.
Photos: Particularly if you take them yourself, you score well on both counts.