Do you mean you want to replace one of the layouts on the toolbox on the left hand side?

If this is what you want then right click on any of the layouts in the toolbox which you don't need. This will show you a list of all the saved layouts. Select your customized layout. To add an icon, make one in photoshop. Icon file size should be (not sure) 30x 30 pixels in bmp format. To add icon again right click on your layout in toolbox and click on change image.

Mahesh