Another topic that comes up often in the Daz Forums is adding backgrounds to 3D images. While I wrote up a quick guide to my views on the ways to do this in the Question about skydomes and other types of backdrops thread, I thought I would expand a bit here to discuss what I see and the strengths and weaknesses of the various options. I link to a few images where I have used these techniques
HDRI images
HDRI Images are easy to use in Iray rendering (and not terribly difficult anymore in 3Delight) and probably the most common way right now to add a sky or background to your image. They have a few advantages in that they completely surround your scene, provide lighting which automatically matches your background (no more shadows going left in the background when the lighting on your figure casts a shadow to the right), and often include a ground texture for a full environment. The drawback is that the quality of the HDRI will drastically affect your experience with them and you are somewhat limited in how you place your figures and props within them since you can’t move the dome. Also they have to specifically be designed to support Depth of Focus (DOF) from your camera; so most of them will be sharp and in focus despite looking like the background is far away from your figure.
If you’re going to use HDRIs from a source like HDRI Haven you will do yourself a favor to get used to the options for either Infinite or Finite sized HDRI domes. Also tools like the IBL Master and Iray HDRI Toolkit will help you use HDRIs more easily and get more value from them.
Images like Neko’s Lazy Sunday and Jogging in the Park are only using HDRI images for the backgrounds.
Backdrop
If you want to use the Backdrop feature (Environment Tab (not to be confused with the Environment tab in Render settings) – select an image for the background instead of a color), you can do it, you just need to edit the image you’re going to use to be the size of your render. That will avoid any distortion of the image when Studio tries to make it fit the background.
I used that technique for my Neko in Venice image. The main advantage to the backdrop over the flat primitive is that it is technically “outside” your scene; so you don’t have to worry about it blocking lights. It also “follows” your camera so you can change camera angles to frame it the way you’d like without having to move the plane behind it.
The drawback is that if you change your mind about how you want to frame the image and it affects the cropping or aspect ratio of the background, you’ll have to re-edit the background image to map to the new dimensions.
Flat Primitive with an Image
For flat backdrops, when you have a specific camera angle already setup, using a primitive plane and putting the image on there is the most flexible. You can size the plane so that the image has the proper proportions and move it around to use only the portion of the background that you want in your render.
The drawback is that you have to pay attention that it isn’t getting in the way of any lights or props you have in your scene. Also, if you move your camera, you may need to re-position the plane to align with the new camera angle.
Curved Primitive with an Image
Several environment sets for sale use these. The advantage over the flat primitive is that the background curves around your set so you aren’t limited to only shooting in one direction. This is particularly useful when you’re shooting an indoor scene and want to have something outside the windows but don’t want to be limited in your camera angles.
While it is more flexible to camera angle adjustments than the flat plane is, you could still run into cases where moving your camera means you need to move the backdrop as well.
Skydomes
By this I mean the “old” way we used to do this which was to create a large half-sphere which covers your entire scene and has an image mapped on the inside. Advantages here are that I get the flexible camera angles like the curved primative, I can move and re-size the dome to adjust to my scene unlike HDRIs, and I get the full sky and surrounding environment. Drawbacks however are that I won’t get a ground like with most HDRIs.
Also, these can cause issues when you’re rendering with Iray. For instance, if you set your Render Settings / Environment to use Sun-Sky or load an HDRI sky image thinking it will provide daylight, technically Iray will think there’s a solid dome covering everything; so the light you’re expecting to come from the Iray environment settings will be blocked by your skydome.
This is one of the more common reasons we see people coming to the forums and asking why their lighting isn’t working on an older environment set.
Cylcloramas
For example, the “Millenium Environment” that comes with Studio. These are really falling out of favor. They have an advantage of providing true depth where some elements are closer to the camera than others. And they are lightweight environments on your computer because all of the background items are pre-rendered flat props instead of full 3D geometry, but you have to be REALLY careful about lighting and camera angles or the illusion is spoiled; so they aren’t very flexible.
No Backdrop
Finally, I’d be remiss if I didn’t also mention that many people will choose to render without a backdrop at all and composite their render over a 2D image using something like GIMP or Photoshop. Since those 2D image editors provide other tools as well for postwork, this can be a great option and is a technique I used for my Walking a New Path image in the gallery. If you’re doing this, you’ll probably want to learn a little about Iray Shadow Catchers because casting realistic shadows on the backdrop is the trickiest part to this technique.
Summary
There isn’t necessarily any “right” or “wrong” to any of these options. They are choices that you make as the artist to what fits best with the image you’re trying to create. I’ve given what I see as the strengths and weaknesses, but you will have to experiment a little and choose for yourself.
Karl's Journey 
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