In order to access the same storage account from multiple roles you will need to add the connection string to all roles in the cloud project:
To initialize the client you would write something like this (again, this will be the same on all your roles):
CloudStorageAccount storageAccount = CloudStorageAccount.Parse(
CloudConfigurationManager.GetSetting("MyStorageAccount"));
Whenever your Web Role sends a message to the queue to notify a Worker Role that the image should be resized, simply include the url or the container name + blob path. With this information your Worker Role will be able to fetch the blob and do the required processing.
I suggest you take a look at the official Windows Azure Training Kit. One of the hands on labs does exactly what you're trying to do here with a Guestbook application (uploading an image on a Web Role, creating a thumbnail in a Worker Role, ...): HOL-IntroToCloudServices Ex2-UsingWorkerRolesAndQueues