You can do this with the free version of SharePoint (SharePoint Foundation, formerly Windows SharePoint Services) that comes with Windows Server--no plugins required.
If you simply want to store the images in SharePoint you can upload them to a Document Library via a web service call. If you want to be able to view the images directly in SharePoint use a Picture Library instead. As you can see in the list of SharePoint 3.0 web services you can work with both Document and Picture Libraries. You could even have the WPF application create the libraries if need be.
As Henrico points out in Storing Images on Sharepoint you'll want to consider whether to store the image file itself in SharePoint or have the file elsewhere and use SharePoint to store the path. His post also points out some other considerations on storing images in SharePoint. Note that his post refers to using SharePoint directly, but the same considerations apply.
I'm not sure what to say about security, since SharePoint supports authorization and authentication with its normal web interface and with web services.
I once used a hosted version of SharePoint to store PDFs uploaded from a command line application that were then viewed by a separate application. It worked great for my needs.