My HDRI doesn't appear on Cycles Render I added an HDRI image (or also I tried, with Jpeg) but when I apply it, it only appears a single color. I think it could be from the image, but it doesn't appear completely as it should be.
Your texture has to be an environment texture (node) and mapped as equirectangular or mirrorball (if it looks rounded) in world settings.  

check "world background" to see it in viewport (perspective view, not ortho)