But, when I use <Import New Background... I get that black screen effect! Will I be able to use the <Import New Background function in the future??
I'll try to elaborate.
By using "import new background" you import NEXT background picture, and the editor switches to it. What you see now - is the SECOND background picture. In the editor you can switch between first and second backgrounds by choosing them in that drop-down list; you can also CHANGE any of them if you want. You can also add more backgrounds (up to 5).
However, when game starts, by default it starts with FIRST background. That's why you were getting black screen: you imported SECOND background, but the FIRST one was left black.
If you want to set active background during game, you should use script command:
SetBackgroundFrame (frame_number);
For example, if you want to start with SECOND background active, put this into GlobalScript's function game_start:
function game_start() {
SetBackgroundFrame(1);
Backgrounds are enumerated from 0 to 4 (that's total five: 0,1,2,3,4).
If you want them animate (i.e. cycle over time):
function game_start() {
SetBackgroundFrame(-1);