A picture may appear blurry if WhatsApp can no longer find it on your device or SD card. This usually happens if the picture is deleted from your device or SD card.
Once a picture is downloaded on to your device, it cannot be re-downloaded, so you would have to ask your contact to send it to you again.
Pictures are saved much more reliably on an SD card. In the future, we recommend that you insert an SD card to your device, if possible.
Cheers,
WhatsApp Support Team