Image as background in Blender:
Click Textures button (red & white checker button
 --if can't see it, make the right pane bigger by dragging the black line)
Click World Textures button (under the Textures button there is a small globe,
 smaller than the World button's globe)
Add new
Change type from Cloud to "Image or Movie"
Click Open and choose an image
Scroll down to Influence section and check the "Horizon" checkbox