If your problem is with music software, I gather you are talking about the cover art embedded in your music files as metadata, rather than booklets and backs, which you would be viewing in a file viewer. You just need one file per album for embedding, usually named "folder.jpg" by some unfathomable convention, but if you are like me, you need many more for documentation.
I too have thousands of ripped CDs accumulated over several decades (and thousands more yet to be ripped), along with vast amounts of album art and documentation. Whenever possible, I buy music as CDs and rip them myself, in which case I sometimes scan the cover and some of the other material myself. But much of the artwork and documentation I have comes from a variety of sources like publishers, online stores, etc.
The ripping software I use (
EZ CD Audio Converter) automatically obtains metadata from various online databases and will download cover art for most CDs I rip, but I almost never keep those images, which are usually inferior to what I can obtain on my own.
To create the cover art file to be embedded in the metadata, whether I scan it myself or convert an existing file, I use
Paint.NET, a free photo editing program that is extremely easy to use but gives me all the flexibility I need.
Artwork obtained from other sources can sometimes be unreasonably large, particularly if it is meant for printing. I'm only interested in being able to read the information, so if the files take up too much disk space, I convert them to .jpg if need be and resize them using
Irfanview, which has a batch conversion and resizing tool with many advanced options.
For adding, extracting and removing cover art from music files, as well as for nearly all my metadata editing, I use
mp3tag.
