I'm glad you reached out about this! Saving an image to render just right can sometimes be tricky -- especially if the version you're working with has been converted to different formats along the way. For example, if an image was created in Word, saved as a pdf and then converted to a jpg it's quality can become compromised. Most of the time when images appear grainy, fuzzy or blurry it's usually due to the file size being too small (even though the image size is set to 600-650 pixels.)
The best possible scenario is to save images directly from the original art file (you may need to ask the graphic designer to do this for you, or ask the person who created the image to send you the original file.) If those options aren't available you can follow these guidelines to prepare your images for the Library.
If you still can't get your images quite right please feel free to email them to us at social_support(at)constantcontact(dot)com and include a reference to this post so we can further assist you.