8700g; Can you change your standby screen picture to a personal picture?

I don't know of any way to set a user-defined standby picture. That's a feature the 7200/7500 series had, which the 8700 lacks, most likely because the screen now shuts off when the backlight goes out.

But there is good news: the standby/keys locked image DOES change when you change your theme! For example, the really cool Overlord Theme (which you can d/l Over The Air from your 8700 by going to Options > Theme > Click Scroll Wheel > Download Themes) changes the standby/keys locked image to a really cool "sci-fi" image of a galaxy, styled to look like an image on a computer monitor in some SF film! And when a call is coming in on the phone, you'll see a similarly-styled image of radar/satellite dishes.

So, if you're lucky, you'll find a cool theme you like, and it'll have a complimentary standby/keys locked image that you'll like.

P.S. Welcome to the forums! Hope you enjoy your stay.

On my 8703e:
I just go to the pictures> then select the picture that I have saved. Click on that picture and then select set as home screen image.