We've noticed that the images that most appeal to us are photographs (rather than diagrams or sketches, though those can still be useful in a pattern's description field), and include at least one person in the picture. We also like images that have a range of colours.  Some patterns lend themselves to more literal representations (e.g. Breaking Bread Together), while others may call for use of metaphor (e.g. Naming).

Some current favourites appear on image ready.

Criteria to apply to each image:

1. Beauty:  Evoke "the quality that has no name."
2. Fit, Relevance:  Clearly pertain to the particular pattern they link with (specifically, they don't confuse the reader to the point they are distracting).
3. Looks good printed at small card size. 

Criteria to apply to the set as a whole:

1. Diversity of demographic (gender, age, ethnicity, group culture, etc.) and setting.
2. More pictures of people in deliberative-type groups, fit our domain.
3. Overall set of images works well together as a coherent whole.
4. Significant proportion of images overall should portray groups of people.
5. Significant proportion of images overall should be beautiful.
6. Significant proportion of images overall should be a good fit.