It seems that the sociable plug in will allow you to show an alternate image like the share this image.

but I don't know if it works properly that way. Can anyone enlighten me about this? I actually wanted to use the share this widget, but it was too complicated and couldn't get it to work correctly or maybe it was that I could only find the image and not the actual widget/plugin. sorry can't remember now.

Thanks
Laura Simmons

This is how I did it.
1. First you need to sign up with sharethis.com.
2. Then Get the button, select your button options.
3. Select the WordPress version, you are then prompted with a download link for the plugin, and the javascript code for the button.
4. Download, install and activate plugin, select 'no' for displaying at bottom of posts and pages, enter javascript in box, save settings.
5. In the Theme options > Post/page Info items, add '%sharethis%' to the post footer or wherever you want it, save settings.