I’ve seen a lot of cool still and animation renders that use a background image. I’m wondering if anyone knows of a tut out there for how to seamlessly put one in. There has to be a compositionally correct way to do it. I’m having a lot of trouble visually removing the dividing line.
To get rid of a seam, you can bring your image into your image editing program and use the Offset tool to offset the image by 50%. Then use your tools, such as the Clone tool in Photoshop to fudge out the line, then re-offset the image so it is back to normal.
In Blender, one of the best ways is to map the image to a sphere. Create a sphere and invert the normals and turn off Double Siding. Add the image as a shadeless texture. Be sure to apply a spherical correction filter to the image if you use this method.
Another way is using AngMaps, or HDRI in YafRay. M@dcow made a bunch of them here: https://blenderartists.org/forum/viewtopic.php?t=24738. Scroll down to the round image that looks like a scene mapped around a chrome ball. After you have the image, go to the World buttons, in the Shaders (F5) section, and add a new texture. Turn on AngMap (it may read as AngMa). Click the Map To tab, turn off Blend, and turn on Hori. Go to the texture buttons. Make sure World is the selected button in the Preview panel. Load the AngMap image.
I hope this helps. 
-Laurifer