What size should a background image be for Apple Configurator to place it on iOS devices?
Your probably best of if you crop the photos yourself. Here are the pixel counts that work best for each screen size:

iOS devices: display sizes


320×480 pixels : iPhone (1st generation), iPhone 3G, iPhone 3GS, iPod touch (1st–3rd generation)
640×960 pixels : iPhone 4, iPhone 4S, iPod touch (4th generation)
640×1136 pixels : iPhone 5/5s/5c, iPod touch (5th generation) Retina display
1024×768 pixels : iPad, iPad 2, iPad mini (non-retina)
2048×1536 pixels : iPad Air, retina iPad mini and iPad (3rd–4th generation), all of which have Retina display


Automate resizing using Automator

Automator let’s you easily crop images. See my screenshot below for explanation and just click the actions together.

You can automate resizing the photos with Automator, if you don’t demand to choose an extract of your photo.



Rember to use “crop images” with “scale before crop: Scale to Short Side”:


After finishing your workflow you could add a thousands images and let your Mac do the work. You will be prompted to select your images (see first action).

Just adapt the sizes for your corresponding device. You could also merge this into one workflow. Just duplicate the last three steps and change the suffix (_iphone5) in action “Add text”.

Here is an example image* went through my Automator workflow.

Original:


Result:


Resizing using an application

You could use the free app Ensoul Wallpapers from the Mac App Store. This let’s you choose an image extract.

The maker of another app (deko) documents the sub-pixel shifts that parallax changes can have at http://dekoapp.com/parallax/



I hope my answer helps you with your problem concerning your image size and how to resize them. Unfortunately, I do not see an option to set a background image in Apple Configurator (Version 3.5 (289)).

* image by interfacelift.com