Bit confused as to what you are trying to achieve. A banner is generally regarded as a large image at the top of the page whereas a full page background is literally that - there all the time with your content scrolling over it. You seem to use the two interchangeably.
With the IRIS theme, if you have a background image it will largely be obscured by the page content which is a solid colour - see for example the ‘options’ page for the IRIS demo. Hence a background image for this type of theme will need to be abstract rather than a definite image.
If you want the page background to be an image with your text / photos etc sitting on top you will need a theme that supports such an approach. Henk Vrieselaar made several such themes and although now ‘retired’ from RW, his work is continued by https://www.themeflood.com - you can see some inexpensive examples there including Blow up, Fluid and Foton - all of which I think have a semi transparent content area. XXL2 also supports this approach.
You need to ensure that the page is responsive in a way that maintains all of the image on all screen sizes. Some are better for abstract images due to the way they respond.
Hope this helps, do follow up if still unsure.