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That’s because your theme adds a 21px gap at the bottom of images.
Look for .tadiv-type img on line 2156 of the styles.css file.
Check with your theme maker for a way to avoid that.
Probably you’ll need to add something like .tagdiv-type .b img { margin:0; } to your custom css.
On top of that, the gap is made bigger by the 10px (top and bottom) margin you set in the group settings in AdRotate Banner Manager.