Watermark
BasicWatermark
The Watermark post effect lets you add all sorts of information to your rendered image such as copyright information or details regarding the render process such as render time, render settings, etc. Enable the Post Effect and RGBA Image Multi-Passes if you want to separate the watermark (incl. alpha channel) from the rendered image.
Generally speaking, a text box (at top or bottom) and any bitmap image can be added.