You use "Spoiler", when an image shows something that would spoil the plot of the anime series or game for users who haven't seen/ played it yet. The image gets covered with a "spoiler" message for users who are browsing the board. They can choose to click this image to display the image.
When you're not certain if an image abides by the rules of Shuushuu, you place it under "review". When you do that, the mods will get a message and can vote whether or not the image gets to stay. While the image is under review, regular members cannot see it until it gets approved.
I hope this makes the difference more clear for you.
Click here to feed me an onigiri!