Not sure this is a good idea or works (just typed it in here) but it should get you started.

You just need to save the current index of the image you are showing in your backing bean (which should be PAGE scoped) and increment and decrement that value.

In the dataGrid

<a4j:commandLink value="Show" 
action="#{backing.setImageIndex(varItem)}" 
reRender="theImage" 
oncomplete="#{rich:component('modalPanel')}.show();""/>

In the modal panel

<h:graphicImage id="theImage" value="#{backing.currentImage}"/>

<a4j:commandLink value="Prev" 
action="#{backing.prevImage}" 
reRender="theImage"/>
<a4j:commandLink value="Next" 
action="#{backing.nextImage}" 
reRender="theImage"/>

In the backing bean

public void setImageIndex(final String imageStr){
   imageIndex = imageList.indexOf(imageStr);
}

public String getCurrentImage(){

   return imageList.get(imageIndex);
}

public void nextImage(){
   imageIndex++;
}