Sure, no problem. I made a very simple widget blueprint to demonstrate.
On the designer tab I basically dragged in an image from the palette to the canvas, named it “Loading Screen Image” set its anchors to fill the whole screen, and zeroed out the offsets. (You could improve this by adding a scale box to keep the image ratio).
Designer Graph:
Then in the event graph I created a array named “Background Images Array”, set its type to hold texture 2d, and added 3 test images to it. Then on Event construct I get the loading screen image and set its brush texture (basically its background image) to a random image in the array. The random int in range node needs to know a minimum and maximum int, so to get the maximum I use the length of the array -1 (because the array starts at 0), this way the random int will always be valid and within the range of images in your array.
Event Graph:
Whenever you create this widget a random image will be chosen and set, of course you could also create a custom event instead of using event construct and call that event whenever you want a random image.
I hope this helps! 