Can anyone explain why when i tick "Use image as stencil" in the transparency panel, then go back to the timeline and place an area emmiter over the video, it puts the emitter everywhere but the character that I bluescreened?
Is Stencil the feature you want to use, or is "Area Emitter Mask" the feature you want? They're similar, but Stencil draws the particles, then draws the stencil image over it, so particles only show where the transparent parts are. Think of it as laying a physical stencil sheet over a drawing -- you only see the parts of the drawing where there are holes in the stencil.
Area Emitter Masks control where particles are created (doesn't clip particles like the "stencil" option does), and only creates particles in the non-transparent areas of the alpha channel.