Well, don't you associate anything with BG?
it could stay for a "Big Girl" or "Bad/Big Game" or just the initials of a friend of yours or a celebrity.
You can also type it in google and see what comes up.
You can practice creativity, you have to stimulate your brain, you can activate both sides of your brain, you can even synchronize them for some time.
Trust me, your brain can do much more than you could ever imagine.
As for creating images, there are no boundaries...images, which work fine for me, may not necessarily work for you as well.
Try to put all kind of emotions in your images.
If you have an image of a flower, you have to smell it, if you have an image of
something to eat (or just anything else) you have to taste it, you have to touch it, feel it.
Such things support your brainm your memorisation.
It's easier for your brain to memorise things if it associates it with something emotional.
Just as an example: when I was eight years old, my first cat died in an accident...I'm almost 21 now, but I still know what I did this day, I know what the weather was like and I know what shoes I was wearing and I know exactly how my neighbour stood at the door to our house and told me what just happened.
I promise you, everyone who was somehow involved in anything that happened at the eleventh september in 2001 will remember like everything from this day.
So, your brain memorises easier through emotion, put that into your memorisation process.
Also, make "normal" images stupid/silly/abstract...
Just as an example again:
you have to memorise BG, your image for that is a Big/Fat Girl...you put that pic on a point of your route. Let's say this point is the toilette in your house (always a good point, seriously

).
Then this fat girl isn't just standing next to the toilette, it's stuck in it, because it's so fat and trying to get out; her face is red because she is so exhausted by that.
I know, that sounds kinda stupid - but that's what it's all about.
You won't forget this picture!
But as I suggested in my last post...make a list by solving the cube.
Do a few solves everyday without the timer. Do them for the images sake...you will get about 5-10 new images with every solve.
Soon you will have a decent list of images and you will reach the point, where you only have to create 2-5 new images a solve...and you will have a complete list sooner or later.
Important things though:
If you use an image for a letter combination: write it down and use it again the whole, so you get used to this image.
You have to avoid similar images.
Let's say you have BG=Big Girl and FG=Fat Girl (this combination won't come up though

) - this will be confusing. Don't do that, you have to find a different image for one of the combinations.