The WordPress ImageShare plug-in gives you the possibility to add social bookmarking icons on an overlay on images in your blog posts.
The textoverlay and color overlay can be seperately enabled/disabled in the admin panel, by (un)ticking the boxes next to "Enabled".
Of course! The configuration file for the icons is located in '[plugindir]/config/icons.php'. There, you can add your own icon according to the standards (which you can derive from the standard icons). Make sure to upload the social bookmarking images in the icons images folder '[plugindir']/public/img/icons/[size]/[index].png'.