I cannot reproduce what you describe here: hitting apply changes the desktop immediately. I've tested with nouveau and nvidia drivers.
This is a wild shot, but...what graphics card do you have and what drivers do you use?
It could very well be a graphics issue. I've had some recent weirdness in other Mint versions, as well, although not this particular issue. I wish I had aimed to buy a computer with Nvidia graphics, which is what I had before. I never ran into problems with it and Linux Mint made it easy to manage the drivers.
Anyhow, this new computer says on Kinfocenter:
Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller: ASRock Revision 09
Driver: Tungsten Graphics Inc.
Renderer: Mesa DRI Intel (R) Sandybridge Desktop
Open GL/ES version: 2.1 Mesa 7.11.2
Kernel Module: i915
Since I don't know what I'm talking about, it may not be what you were looking for, but really, I don't want you to spend time sorting out my graphics baloney. I love what you've done here. The system is working fabulously well. I rarely change desktop backgrounds, and even if I do, this is not a big deal at all. Please don't worry about it anymore.
PS: My computer has been seeding all of Saturday and overnight. Now it says the transmission is "Stalled" and I was wondering if I need to restart when it says that?