A flexible Mega Panels layout works best when images are properly sized. An example use-case would be having three panels in a row, each with an image that fills the width of each panel.
In order for the image to fill the space allowed in the panel, create the images with the correct width to correspond with the size of the panel.
The Mega Panels module bundled with Luggage allows site builders to create Panel layouts and place them in blocks.
Example Mini Panels
In this example a Mini Panel block contains a Mega Panels layout. Each pane contains a Custom Block with a link styled as a button.