The best practice is to use images and/or video clips that are the same size as or larger than the video you are outputting. 


The video sizes are listed in the quickstart guide.For example, if you are using the High quality setting, the video output is 1280x720. Photos that are 1280 wide by 720 tall, or larger but with the same 16:9 width to height size ratio, should come out perfect.


If your images are coming out blurry, they are likely smaller than the video output and are being stretched. That results in a loss of quality.