I want to put an image in the sidebar of my blog. The picture I want to use is on the hard drive of my own computer, and the image widget wants a URL. Is there any way I can place this image in the sidebar in more or less the same way I place an image in a post?
Upload the image via Media > Add New, copy the File URL, use that URL in the widget.
If the image is wider than your sidebar, you must first edit it in an image editing application to downsize it. For maximum width see here: http://wpbtips.wordpress.com/2010/01/11/sidebar-width/