You can add background images either by using environment settings, capping window holes, or by creating large flat objects in the background.
Generally, it is recommended to use the direct/reflect/refract environment overrides in Render Setup > Scene > Scene Environment.
See: How to set up HDRI environment?
But sometimes you may need to use a different approach - for example if you are not using a single flat or spherical background image, but a set of images that will be visible outside of each window, or if you want to tweak the mapping of your background image. After creating the background geometry, you need to assign special material to it:
Create Corona Light Material with "Emit light" option turned off and load the background image as Texmap - this method is easy and fast to set up. You can additionally toggle direct, reflection, refraction, alpha channel and masking element visibility for such materials.
If you need additional controls, for example to make reflections or refractions of the background image darker, instead of using Corona Light material you can assign Rayswitch material to the background geometry:
1. Create a Corona Rayswitch material (let's call it A)
2. Create a standard Corona material (let's call it B)
3. In the standard Corona material (B) disable diffuse component (set diffuse level to 0 or colour to pure black)
4. In the standard Corona material's (B) self-illumination slot, put the image you want to use as the background - use self illumination multiplier to set the brightness of the background image
5. Put the standard Corona material (B) into Corona Rayswitch material's (A) "reflection", "refraction" and "directly visible" slots
6. Leave "Global illumination" slot empty!
7. Assign the Rayswitch (A) material to your background object
This way you will create a material that casts illumination, is visible in reflections and refractions, but does not affect your scene by casting shadows or generating GI. To change the appearance of reflections or refractions, simply alter the materials used in "reflection" or "refraction" Rayswitch material's slots.
Regardless of the workflow of your choice, to load the background texture you can also use Corona Tonemap Control map to make it unaffected by color mapping (EV value, highlight compression, etc).
Examples
1. Corona Sun + Corona Sky - no background image, no geometry capping the window holes - Corona sun and sky generate lighting and are directly visible.
2. Corona Light Material assigned to planes capping window holes - "Emit light" disabled. Lighting is still generated by Corona sun and sky, but the background image is visible directly, in reflections and in refractions. The image is in "screen" mapping mode.
Material setup:
3. Environment overrides in render settings. Lighting is still generated by Corona sun and sky, but the background image is visible directly, in reflections and in refractions. The image is in "screen" mapping mode.
4. Corona RaySwitch material. Lighting is still generated by Corona sun and sky, but the background image is visible directly, in reflections and in refractions. The image is in "screen" mapping mode.
Material setup - the rayswitch material:
Material setup - the self illumination material:
5. Corona RaySwitch material. A slightly darker material was used for reflections and refractions. Lighting is still generated by Corona sun and sky, the brighter background image is visible directly, and the darker image is visible in reflections and in refractions.
Material setup - self-illumination was decreased in the material used for reflections and refractions: