I am not sur to have well understood the topic (I am french) but is the rendered image blurred or pixelated? It is not the same at all.
I had a similar problem, and it was not at all a problem of picture format or quality!
On a simple image, I had the Filter parameter of the Texture-Image panel set to a value superior to 1.0 and the result was a blurred texture.
In a complex scene it is easy to see wher the problem comes from, but in your example (a simple object) you can believe that it is a rendering problem instead of a texture parameter problem.
Am I out of the topic?