Stumbleupon for me is ultimate place to find really interesting stuff. I use it on a daily basis to get inspired and find new stuff around. Stumbleupon users like all other social media users like Images, but when discovering a article from some themes/sites, it picks a wrong thumb, usually of a ad. It depends on the theme structure.
That is where this simple trick comes in handy. Before discovering a article from your blog (only where Su picks the wrong thumb), add below code to your post (after modifying it accordingly) and it should pick the correct thumb.
One Step Tutorial:
After publishing your post, Edit it and add below code with corrected links to THUMB and ARTICLE link:
document.write('<div style="position: absolute; top: 20px; left: 50px; background-color: azure;"><img src="'+sImageURLz+'" height="'+imgHz+'px" width="'+imgWz+'px"></div>');
Note: Please change var sImageURLz=’ and var sURL2z=’ with your Thumbnail link and Article link respectively.
After Discovering the article, you can should edit the post again and remove the code added.