Mouseover gallery divids the content into a left_part and right_part (classes) where the thumbs are put into the right_part and the enlargments comes
What images does the gallery use?
The plugin does not use the default image you upload, instead it uses the Medium and Thumbnail versions of it. It is done in order to produce better image quality and to increase performance. The size of these versions are set at Settings > Media. And why is it important? Because if you are running a photo blog or simply plan to use a very big gallery you may need to update the size of you Medium and Thumbnail versions of the image. The default sizes are 300X300 px and 150X150 px respectively. If you plan to set the main image to 960X700 px and/or use larger thumbnails it might be a good idea to set these sizes at the Settings > Media as well.
How do I add galleries to posts?
Remember, Mouseover Gallery uses the default WordPress Gallery feature.