Only a few steps are required when saving your images for CommandPanels to find them for a custom map.
You will need to create a new folder and name it 'maps' as seen in the image below. Within that folder, you can place your image there, give it any name you would like:
Once the image has been placed in the 'maps' folder, you can now use the image in your Hotbar Item. You will need to ensure the item material is FILLED_MAP and that a new section is created called 'map: map1.png' while using the image name you have created. The image below illustrates how the item should look. Note, the map image files should be 128x128 pixels. If they are larger Minecraft will render them too large and if they are too small they will not fill the borders of the map.
The first and easiest step would be to simply drag and drop a custom banner into the in-game editor. Otherwise, simply add a banner section to the panel item like so:
'1':material: RED_BANNERbanner:- WHITE,STRIPE_MIDDLE- BLACK,SKULL
Using the PatternTypes there is a list of different patterns that can be used, the order will be read from top to bottom for which pattern will be placed on the banner. The list can be expanded as long as you would like, as they will be in this format: