I am using the SharePoint client API (javascript) and jquery to enhance the out of the box SharePoint list view functionality.

I've used [qTip][1] for enhanced mouse overs ([it supports images too][2]). I was able to meet my requirements with it.

The code snippet below should get you started.  Check out this reference to fetching files from within folders using the javascript and the client API ([click here][3]).  If you're using Sharepoint 2007, [this post][4] will help.


    
    function bindQTip()
    {
    
    $('.ms-vb-icon img[title*="Folder"]').each(function () {
        
        	//bind the qtip event
        		            $(this).qtip({
    	                position: { corner: { tooltip: 'topLeft', target: 'topLeft'} },
    	                content: {
    	                    text: 'Loading...',
    	                    title: {
    	                        text: 'Recent Images'
    	                    }
    	                },
    	                show: {
    	                    solo: false,
    	                    event: "mouseover",
    	                    delay: 250
    	                },
    	                hide: {
    	                    fixed: true,
    	                    event: "click",
    	                    delay: 250
    	                },
    	                style: {
    	                    width: 100,
    	                    padding: 3,
    	                    background: '#f0f0f0',
    	                    color: '#000000',	                    
    	                    tip: 'topLeft'
    	                },
    	                api: {
    	                    beforeShow: function () {
    	                        setContent(this);
    	                    }
    	                }
    	            });
    					
        	  });
        }
    
    
    function setContent(currentNode)
    {
    	//Call SharePoint API to fetch folders / images and build new qTip content as HTML
    	
    	//Use qTip .updateContent() API method to set the item content as HTML 
    	var itemContent = "<div id='imag'><img src='/_layouts/images/siteIcon.png' /></div>";
    	currentNode.updateContent(itemContent);
    }



  [1]: http://craigsworks.com/projects/qtip/
  [2]: http://craigsworks.com/projects/qtip/demos/content/images
  [3]: http://www.learningsharepoint.com/2011/09/07/get-files-from-a-folder-ecmascriptjavascript-client-object-model-sharepoint-2010/
[4]:http://stackoverflow.com/questions/6351845/sharepoint-list-getlistitems-webservice-to-return-sub-folder-contents-recursive