On **some** hardware Peppermint 7 has graphics issues during the install on recent versions of Virtualbox .. The upshot is that when you boot the Peppermint 7 ISO virtualbox jumps to a strange cinema scope resolution and the graphics are all scrambled.
This is NOT a Peppermint specific issue, at least Lubuntu 16.04 also does the same thing, and other distros are also probably affected.
Once installed in the Vbox VM it runs perfectly, but you may need to make the following small tweak to get it installed in the first place.
In the VM, boot the LiveCD, and when you get to the "Try *ubuntu" or "Install *ubuntu", hit F6
now hit Esc
Now you should see some white writing at the bottom of screen that ends with:-
quiet splash ---
change that so it ends with:-
quiet splash --- vga=791
and hit enter to boot.
Now GRUB will be forced to stay at 1024x768x16 and the VM should boot perfectly .. you can then install Peppermint 7 as you normally would in Virtualbox and it should work as expected