Looking at the image, it looks like you intend to use that image for the scene in some sort of platform game.
Yes. You are correct. What I'm thinking is that your character will be moving around on top of the green bits and that he/she will be able to move through the blue bits (the "sky")
Again, you are correct. Then to detect if your character has collided with the green bits of your image, you can just use a simple:
if imagescollide(map, 0, 0, 0, character, charx, chary,charframe)
endif
I don't understand what to put between the If and EndIf. What would I put to make it so that the character doesn't go through the green parts? if imagescollide(map, 0, 0, 0, character, charx, chary,charframe)
endif