On the Recent Posts Plus widget, I've used the Widget Output Template to place the thumbnail under the post title. On "normal" pages, archives and posts, the widget works like a charm and the image is sized according to my settings.
However, on any of the shopping cart pages, the thumbnail is one of the default header images delivered with TwentyEleven if full resolution. Every time the page is refreshed, the image changes. It also appears above the post title.
WP e-commerce uses the "global" WP_Query on these pages, which may be conflicting with the recent posts widget.
This is on a network site, but I don't think it matters.