Image Panels are added the same way other controls are added.
On iPhone, in Design Mode press and hold the + icon for choices and choose Image Panel

On iPad, it's the last choice in the Add Control dialog
Image | Position on the first pane of the Design Properties Dialog gives all the options for image choice and position:
Adjust transparency using the Alpha slider. Higher numbers mean more opacity. By default, the Image Panel is fully opaque (alpha = 1.0) but you can change the alpha to anything from 0.3 to 1.0.

Z-Order: Back to Front Layering
By default, all Panels (Image Panels and Panels) are always under other controls. To turn this off, switch the Always in Back toggle to off (not blue).
Once this is disabled, Image Panels can be in any position from front to back. If you need to change the order of any control, you can do one of the following: