How to insert album cover art on music files?

I have searched the net for answers and I have not found any feasible answer to the question:
"How to insert album cover art on music files?"

I have about 4-6TB of music on external hard drives. Much of this music came from live broadcast sources through the years. I also have much HR music recorded from various on line services recorded via a HR Tascam recorder then transferred to the hard drive.
How can I insert a picture into those music files on my hard drive? Low rez sources such as iTunes, or mp3 conversion is not an option.

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I’m going to look into the suggestions. 
So, I will be able to just plug the hard drive into my computer and then add artwork to files I want?
Or, do I need to add all the info from the hard drive into the downloaded program first?
I ask this because I think that is what Music Media required.
First, it is so time consuming and second it will probably overload my computer.

As you can tell I am not of the computer generation, but trying.

Is it just called Meta? When I do a search I get all kinds of crazy results.

I use Tag&Rename to tag all my WAV files. Never had a single issue with the information/album art showing up using the old Slim Devices/Logitech streamers or my current DIY computer using JRiver.
I have dBPoweramp and Mp3Tag.

Mp3Tag is (to me) slightly more user friendly. It cannot be easier. It is free, but it asks you to donate if you wish. I do about once a year because I do use it often.

In case you decide to use Mp3Tag, remember to delete old picture AND save that deletion if you want to change the picture on the file. Only then add a new one. Do not worry, it is about 1 second of extra work.

For me, WAV covers seem to be more related to the machine it is played on than to the program it was tagged with. Same file will show on some of the machines and not on others. Even from the same brand.

dBPoweramp converts pretty much any format I use. I could not discern the difference between FLAC and others.
kgturner, glupson,
Can I just pick the files on my hard drive that I want the art work added?
Can I just cut and paste the pictures to go on my hard drive?