Split document library

Root problem: you are hitting the List View Threshold (LVT) (5000 on SharePoint Online).

Tools like "Open in Explorer" and many other tools cannot cope with this because SharePoint blocks requests that consume too many resources. 

Do you want to keep the pictures in the same library? If so, downloading them and then uploading them into a folder structure will be quite cumbersome.

There are tools that can circumvent the LVT. For example SLIM Companion Explorer retrieves data in chunks. The tool mimics "Open In Explorer" in a browser. You can use the tool to move pictures to folders within the same document library or download all items. It also offers previews for images similar to File Explorer.

One feature may also be of interest to you: it is capable to extract metadata from images (and also other formats like emails, PDF, ..) during uploading and capture the values into SharePoint columns. This way metadata like date taken, GPS location, etc ... can be used in views.

Note: I am affiliated with the vendor.
Paul