Actually since you're within Visualforce there's a much easier way of doing it. Assuming that you have a Controller variable sig equal to the name of the signature's StaticResource, e.g. 

public Opportunity theOpp {get; set;}

theOpp = [Select Id, Owner.Signature_Image__c From Opportunity Where Id = :controller.getRecord().Id];

String sig = theOpp.Owner.Signature_Image__c;

Then you can dynamically access the Resource like this:

<apex:image value="{!URLFOR($Resource[sig])}"/>

 However, I would NOT use StaticResources to store Signature Images, I would use Attachments. Attachments are much better suited to this purpose. For instance, assuming that the Signature_Image__c field on the User object contains the Id of an Attachment containing as its body an image of that user's signature, then you can do this on a Visualforce Page with the Opportunity StandardController to display the signature:

<apex:page standardController="Opportunity">
<apex:sectionHeader title="{!Opportunity.Name}"/>
<apex:image value="{!URLFOR($Action.Attachment.Download,Opportunity.Owner.Signature_Image__c)}"/>
</apex:page>

or, if you already have the image as a variable, e.g. signatureAttachmentId, in your controller, then:

<apex:page standardController="Opportunity" extensions="OppExtensionController">
<apex:sectionHeader title="{!Opportunity.Name}"/>
<apex:image value="{!URLFOR($Action.Attachment.Download,signatureAttachmentId)}"/>
</apex:page>