about 20,000 images

Definitely too much to upload each one....

how to upload them to steemimages.com for hosting

The upload must be signed. I once saw the code for this in the condenser code, but only at react level.
I don't know exactly whether it also works with the Steem-JS library (https://github.com/steemit/steem-js).

Maybe this will help you: https://developers.steem.io/services/imageHoster
Among other things, the upload process is described there.
Alternatively, you could of course use your own image hoster. There was a repository for this from steemit, but it is no longer available...