They say that a picture paints a thousand words... I think that most Business Analysts would agree that a good process flow or use case diagram can provide a visual aid that can summerise and explain requirements more concisely than words.
This brings me to the Aotea Studios blog, this blog provides free posters and articles for business analysts and project managers. The blog covers many areas of business analysis and project management.
I suggest you check it out. And if you find that they don't have an illustration for an area you are looking for... ask them for it!