From the left-hand Admin main menu, select “Media” this will bring you to the Media Library. If the image you need has already been added to the Media Library, you can search for it using the “Search Media” field to the right side of the page. Tips for searching:

• to search for a collection item image, try using the item’s accession number; use underscores rather than dots or hyphens: “L00_084” not “L00-084” or “L00.084”
• if using text to search for something, start with distinctive partial words or relatively general words. For example when searching for “Account Book of Elijah Williams, Ledger B, Vol. 3” try searching for “Ledger”. You’ll get several hits from which you’ll need to choose, but you’ll avoid getting no hits and wondering why which is what might happen if you searched for “Account Book of Elijah Williams, Ledger B, Volume 3”

If the image you need is not yet in the Media Library, you’ll need to upload it from your computer. To do this click the “Add New Media File” from the button in the upper left part of the page:

This will let you browse to the file on your computer and select it to upload to the Media Library. Once your image is in the Library you can select it to insert into your page.

For more info, here’s a link to the WordPress documentation about using the Media Library