What's wrong with the thumbnails in the recent post?

I can't figure out how some of my post have a thumbnail in the list of my recent posts while the majority doesn't have. *sigh* It's so an odd sight seeing my posts without a thumbnail. I don't know if it can affect the number of visitors but I know it can be a factor. I ain't an html-techie or an expert. I still hope I can fix this out.