In order for an HDRi sky to be used as a background, it need to be a very high resolution image, as otherwise it will appear pixelated, since you are seeing only a small section of the image.
For example, if you use an HDRi image that is 4000 pixels wide, and you are rendering with a 30 degree field of view, then the part the image that you are seeing in the background of the rendering is approximately only 333 pixels wide. This is beacuase 30 degrees is only 1/12th the full 360 degree HDRi panorama. So that small part of the image (4000/12 = 333) has to be enlarged to fill the size of your rendering.
Most HDRi skies that are free, such as the ones we include with the IRender nXt install, are not high resolution images.
You can look at the list of HDRi file sources on our web page:
to find other HDRi skies, but that list may be somewhat out of date. You might just want to do a search on the web for high resolution HDRi sky downloads.