I have just read a question on how to calculate Signal to Noise Ratio (SNR) here, and I was just wondering how I can relate SNR to the image quality. More specifically, I want to know what kind of SNR level can we tell that the image is of bad quality.
There are several image quality metrics - most closely related to SNR is PSNR (peak SNR). You can read more about it here.
Of interest to your question:
"Typical values for the PSNR in lossy image and video compression are between 30 and 50 dB, where higher is better. Acceptable values for wireless transmission quality loss are considered to be about 20 dB to 25 dB."
In general though, PSNR is only an approximation to human perception of image quality.