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.

Technically, no audio degradation if done lossless at same resolution and a good quality streamer that works with flac format is used. Try it and see. There is always a chance some streamers do better with one format than another Even if source music content is identical, but should not be the case if done well. Some report differences case by case but has not been an issue for me. I have thousands of releases in my flac music library that I stream all the time and sound quality is never an issue.   Many were converted to flac from wav using dbpoweramp batch converter.  I’ve ripped exclusively Directly  to flac though in recent years

I've looked at Picard and it's clutzy. Mp3tag free, drag 'n' drop in any art you want. Manage tags easily for folders.
Batch conversion I recommend XRECODE3. It was free 10 years ago. I donated then because it was so good. Best $15 you'll ever spend. Or €10.
I use a program called Meta. I think I paid $20 for it, but it's super easy to use and does the job. I use it to add album artwork and meta data to my wav files.
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.