Try using a ClientBundle with ImageResource
in the Common project. This will make it very easy to reference that bundle and all of its resources, so that you don't have to worry about relative file paths when trying to pull images out of it.
I like to have a resources package in a given widget's package. So you'd have the following structure:
com.yourCommonStuff.yourModule.client.widgets.yourWidget.resources (ClientBundle interfaces here)
com.yourCommonStuff.yourModule.client.widgets.yourWidget.resources.images (image files here)
com.yourCommonStuff.yourModule.client.widgets.yourWidget.resources.css ( css files here )
com.yourCommonStuff.yourModule.client.widgets.yourWidget (other widget stuff here)
This way everything for a widget is nice and self contained, rather than a bunch of widgets dumping all their CSS and images into a single public/ folder. It makes referencing files far easier, otherwise you end up with lines like @Source('../../../someOtherPkg/public/images/image.png').