Hi Bryan Ogle,
As per your description, yes when we add image in Microsoft list via click on Attachments or add a “Image” column in Microsoft list and by default (for selecting an image) it will navigate to windows explorer on your locally drive in Microsoft team channel or in SharePoint online list.
Since, you mentioned, “I want to open a specific document library (in the same Team's channel) for the user to select the image from” what you can do is, use OneDrive sync client app to sync your specific SharePoint online image document library on your pc then you navigate to sync SharePoint library in file explore and see if this meet your requirement
I appreciate your understanding and stay safe!!
Best Regards
Waqas Muhammad