In creating a theme, how can i allow a user to change an image outside post/page content?

I ran into a similar problem recently. I found plug-in called “image widget” http://wordpress.org/extend/plugins/image-widget/screenshots/ that enabled upload right in the widget.

The client though had no idea how to barely log in and find the widgets so I ended up using a flickr widget since it was more user friendly ( aka they could upload the image from their iphone to flickr).