We recommend decks that are at least 1024px wide, and we now support decks up to 1920px wide. If they are not as wide as that, sometimes the quality can suffer. Try increasing the resolution and re-uploading the deck to see if it displays a better quality image.
If your deck is at least that wide and you are still having quality issues, please email support@speakerdeck.com and we'll take a look.
