Pictures you paste in from the Clipboard are pasted in opaque. Pictures you copy by Option-dragging are either opaque or transparent, depending on whether the original was opaque or transparent.
While a recently pasted or Option-copied picture is still selected, it "floats" in its own plane. You can make it transparent or opaque to uncover or cover anything it may obscure.
Immediately choosing Select and then Transparent from the Paint menu after you draw an image uncovers any image it may obscure.