It all depends on what you are tagging and how you have currently setup Picard. Not all releases have cover art on MusicBrainz. In this case it will often help to also enable the “CAA Release Group” cover art provider.
In addition you can enable additional cover art sources in Picard by installing additional plugins. Currently there are plugins for Fanart.tv, TheAudioDB and Amazon.
Both Fanart.tv and TheAudioDB offer high quality cover art. Amazon can definitely be a mixed experience, but I think serves well as a fallback option.
In Options > Cover Art you can configure which sources you prefer. Picard will first try getting cover art from the sources higher in the list, and fall back to the lower ones. E.g. this is a possible configuration: