The quality of the final image is limited by the quality of the image used. In this case, I strongly suggest that when you search for an image, you should include the resolution for the image (i.e. "duck 1920 x 1080"). By doing this, you are making sure that you use high quality pictures for your boards. Also, I suggest that you use the Export function to save your boards. This option will not decrees the quality of the images used.