The picture on the right is a cluster of the aphids on French prunes. The picture below is of the whitened leaf margins that are the diagnostic sign of the pests. The aphids tend to hang out on the bottom side of leaves and are less visible. The whitened leaves and the vast amounts of honeydew on the bottom leaves are easier to see.
If you have a problem with these aphids you shouldn't panic. They're pretty easy to kill with any aphicide that's labelled for fruit trees. Or you can just let them alone for a bit (they hardly ever carry diseases) and let their natural enemies clean them up.