On my bookmark toolbar (just below the URL box) I see an array of bookmarks. Some have their own individual icons while others have a blank/generic bookmark.
Q:
How can I get those blank/generic bookmarks to have a picture associated with them?
Do you mean make some of your own? I don’t think you can do that. Those icons are created by the web designers, they’re called favicons, and are placed in the website folders for the browsers to pick up.
If you think some of your links are missing some that ought to be there, there are applications that are designed to locate them and reset your bookmarks again. I use FavOrg for that.
There may also be apps that do allow you to make your own, though I’ve never seen any.