If you are creating everything with blocks, then I'm afraid there is no easy way to make them show in the scene designer. But you can work around this, just need some extra effort. Some ideas, since I don't know your exact setup:
If your final image is the same size as the scene, you can in photoshop pinpoint the spots where you'd need the terrains, with their dimensions and positions (remember that the top left corner is x: 0, y:0), assuming you have the full image in photoshop. You can set a terrain/region's position and dimensions precisely in the Inspector tab inside the scene designer, so that should be doable.
If you don't have a mock up, and your images are individual pieces, you could test game, your behaviors would assemble the full image, and you could screenshot the whole thing and stitch it together, then do the above process.
Another idea, unrelated with terrains, are the blocks under Actor> Properties > Collision Shapes. You could have an actor created at the position of every one of your image instances that would have a collision, and setup a collision shape to match the image's position and dimensions. I've never used them though, so I don't know if that would suit your needs.
EDIT: And of course, you can create the terrain regions with code blocks, or there might be an extension for terrain blocks in the forums.