I'm looking for an open source or licensable Windows image-picker UI
for use in a graphics application that I am writing. I would like to
be able to customize it and publish it in a commercial application.

Ideally this is a dialog box that I can call to let the user choose
one or more photos from the filesystem. Important user features are
thumbnail preview and easy multiple-selection (e.g. checkbox next to
each thumbnail, or a list of selected photos that the user explicitly
builds). It needs to be significantly easier-to-use than the standard
Windows file picker UI (assume an older audience).

Screenshots of image pickers that I think of as easier-to-use:
Yahoo Image Picker (see link for a picture)
http://help.yahoo.com/us/tutorials/mail/pm/s_selecttool.html
MSN Photo Upload Control 
http://www.microsoft.com/windowsxp/using/digitalphotography/images/share_familygallery_11.jpg

Ofoto, shutterfly, and snapfish also all offer UI's of this sort. (I
should clarify that I only need the picking functionality, not the
upload functionality that is the goal of many of these UI's)

Minimum requirement is Windows XP. Should not rely on .NET framework.

There are many upload tools available which do the part of picker.
These can be used by paring down the upload functionality. (Search
google for photo upload tool). The nice thing is these are smart
browser based tools which can be easily integrated.

Also you have to worry about Java or ActiveX and variety of browsers.
Look at www.photoship.com (demo at
http://www.photoship.com/viewdemo.htm) for such a tool which works on
all browsers. It is a free to register and use site and is a good
example of a Photo upload. (It also supports drag and drop into the
pane).