I believe that this is referring to the tendency for humans to incorporate their own phenomenology (or subjective interpretation) into their perception of an object.
I think that 'modes of thought' in this context refers to the individual way that a person may think about something, independently of what the thing may represent objectively. For instance, think of a flashing red light in any way that you so choose. A flashing red light is always a flashing red light outside of subjective interpretation, but each person may perceive the light differently in his or her mind's eye. One person might envision an omen forecasting danger, whereas another might envision a broken stoplight.
Following this definition, 'loci for modes of thought' would refer to objects that have the property of inducing this sort of subjective interpretation in others. As mentioned earlier in the paragraph, the aesthetic properties of the images are not 'the point' of their appeal. In effect, the science images act as platforms for individual interpretation, to the detriment of their associated overall, 'objective' meaning. In other words, to the average person, they may increase -- rather than decrease -- their associated interpretations and meanings, creating more questions than answers but also lending to their perceived beauty.