Bouldering is climbing on large rocks, called boulders, or very short cliffs fairly close to the ground. Since boulderers climb closer to the ground, ropes are not used. (Sometimes boulderers will use a rope on tall boulders, called highballs, to brush the rock and rehearse moves, before climbing it with a rope.)
Key things to identify bouldering images:
Currently, LCSH maps Bouldering to Rock climbing. Use the term “bouldering” in title and/or description and “Rock climbing” in the subject.
Example Image 1: Karl Dunn in traversing contest, Arches National Monument, Utah
Here you can see Karl Dunn climbing a large rock that is close to the ground. There is a person close to the base of the rock (with head slightly visible at lower left corner), keeping watch.
View full metadata here.
While this video is not in an institutional repository, it contains demonstrations of bouldering.