Aperture does not have layers, is not a graphics package, and does not do anything with chromakey or compositing, and is therefore, as David AM states, not the right software for what you want to do. Any graphics program -- they are designed from the ground up as compositors -- can do what you want. Photoshop is the most famous, but many here use and recommend Pixelmator and GIMP.
Aperture is a photographer's workflow tool for converting (from RAW), organizing, developing (making each Image as appropriate to your needs as possible), and publishing digital camera files.
For combining an image with another image, or with text or shapes or anything else, you need a graphics program. Basic combinations -- borders, simple text -- are available with plug-ins (viz.: BorderFX). Complex combinations require a more robust graphics program. Most Aperture users assign one to Aperture's "External Editor" function.