I think I know what it is… the sidebar column is 370px wide, but your image thumbnails are being resized to only 342px by this slider which is applied as an inline style on each generated thumbnail. Also, your original size of each image is 150×150 which is the thumbnail your Media is being created when you upload images….long story short, they are too small. As for the slider, I am not familiar with it so you may want to double check your settings. Your other option is to try a different image slider. The Widgetkit one I use on the demo site can be used for this.
