Here are three examples of image slideshows on this page that can be used on blogger sites.
For the purposes of differing-shaped images, I think the best slideshow is the Picasa Web Slideshow, which is Number 1 in our list of examples:
Sample 1: Google Picasa Web Slideshow:
This is the best option because Picasa Web kept all images at the correct aspect ratios when they were converted. We didn't have to make any changes to the images in Photoshop. Very cool! And the black background will make it disappear into the background of your blog too making the art pieces themselves the complete slideshow.
Sample 2: SlideRoll slideshow:
Sample 3: ImageLoop slideshow: