You can translate! plots and scenes, but not layout elements. To make the Box and the Label visible you probably need something like
ax, p = image(fig[1, 1], ...)
translate!(p, 0, 0, -1)
If you want more control over your label you could try making it manually.
mesh!(fig.scene, Rect2f(200, 200, 50, 20))
lines!(fig.scene, Rect2f(200, 200, 50, 20))
p = text!(fig.scene, "(a)", position = Point2f(205, 205), align = (:left, :bottom))
fig.scene is in pixel units which is probably more convenient than axis. boundingbox(p) might also be helpful.