Slicing makes it possible to create complex images for the Web. A slice is a rectangular area of an image which becomes a cell in an HTML table in the HTML file for the image. 

While slicing your picture you can adjust each slice.

When saving your optimized document as an HTML file, Image Splitter generates an HTML table and places each slice in its own table cell. By default, each slice contains the underlying image content in the document. Slices containing image content are called Image slices. Alternatively, you can specify a slice as a "No Image" slice, to contain a solid color and no image content. (Because "No Image" slices contain no image data, they download more quickly.) You specify options for slice content based on the slice type.

All new slices are Image slices by default. 
With "Image" and "No Image" slices use Testing button for controlling slices appearance.

Note: Image Splitter does not display "No Image" slice content. To view "No Image" slice content, preview the image in a browser (View->Testing).