Welcome to the forum.
As both Premiere Elements and Photoshop Elements share the Organizer, you might get the answer to your question here, but since you are talking about Still Images, I would post to the Photoshop Elements Forum.
Thank you so much. I did not realize I was on the Premiere Elements. Love your picture!
Catalogs are the files in which the Elements Organizer stores references to all of your media files, keyword tags, albums and other data.
I've never heard of a catalog being so large that it slows the programs down. Some people have hundreds or even thousands of photos in a single Organizer catalog and it still operates quite efficiently.
In reading more from the actual software program under File,Calalogs, is where I ran across the note about having too many in a catalog. AND, this should have been posted under Photoshop Elements. I don't know if it works the same or not. Thanks.