The right margin, which is the one a left-to-right reader leaves rather than the one they return to. A picture there is glanced at on the way out of the line instead of on the way in, and it interrupts even less than the left — which is either what you want or the reason to use the left.

Everything else is identical. The float is pulled clear by its own width plus the gutter, so the copy keeps the whole measure and the picture keeps its full size.