Wow, this guy is really good, and has really good tools.
It's incredibly hard to work panties/pasties or other modesty fig-leaves into a whole-body painting. The entire point of the form is to let the human body, that we are evolutionarily primed to react to viscerally, underlie the artwork. While a canvas is often supposed to be invisible, so that you "see" only the image on it, a body painting uses the flesh - see the "Predator" animated .gif on the imgur page for one example - and fig-leaves jarringly break up the interaction of perceptions that make a good bodypainting more than just another excuse to look at an attractive unclothed body.
Unfortunately, one often has to use fig leaves in order to get co-operation from a model or venue, because painted naked bodies are undeniably erotic, at least to some portion of viewers, and some people don't like that. You can see in Mr. Roustan's work that he doesn't always entirely "hide the panty line" so to speak, but you can also see instances where he's worked in the contours of the modesty garments in the same way that he's worked in the muscles and hairs of the models, so that they are a cohesive part of the effect. I'm very impressed.