Oh, please Google, stop, for the love of search, make it stop!
I can’t see an option to remove the background image and switch back to the default. I tried replacing it with a tiny transparent image, but I was denied …, as it requires an image at least 800×600. So, I tried a white 800×600 image:
Weak though. I liked the fact that the image was tiny (around 600 bytes), but it’s too washed out. Apparently, Google doesn’t have an algorithm that can make a smart decision about text contrast and the photo being used. Too bad.
So, I tried a black image:
Not too bad. Passable.
I’d prefer however an option to switch back to the simple white background. The default background image today is absolutely awful (IMHO). I’m sure I’m not the only person this morning shocked by the change.
This option is such a tease:
As it just removes an image you selected, and switches back to the default image. 
If you’d like the black background I use as a temporary fix…, just click on the black rectangle below, which should open the original file. Right click on that and “save as” to your desktop.
Today, I will start using Bing more. I like the way they do background images. Hand picked. Not too large. Easy to read text…
.
workaround, visit https://www.google.com or http://www.google.com/ncr. That works, but it still sucks