I'm trying to change the image for the dialog "SetupType" (see attached - I circled in blue what I'm trying to modify). I went into edit mode but there's no control on that dialog that can let me browse for an image (see attachment 2)
I'm using InstallShield 2015 Premier Edition Version 22 SP1
I have 2015 Professional not Premier, so your mileage may vary. Custom images in the banner are not supported per se (that I know of), but neither are they explicitly verboten (that I know of).
Go to C:\Program Files (x86)\InstallShield\2015\Support\Themes or the equivalent on your system and find the folder for the theme you are using.
Backup the original images in that folder to somewhere. Then you can edit the images, but I would be sure to keep them the same size in pixels.
In your 2nd screenshot you seem to have created a new text control rather than select the "Banner" bitmap control. If you select the Banner control then you will be able to provide a path to your replacement banner image.
You might have to 'tab' around a bit to get the correct control selected...
