As the rules state:
Artwork. Literal photographic representations of the entirety of existing works of art (including your own) are not considered acceptable submissions, however creative depictions or interpretations are permissible. This includes, but is not limited to paintings, sculptures, photographs, drawings, and computer artwork.
As you can see, photographs and computer artwork (including your own) are allowed as part of the scene you create. A photo manipulated would be considered computer artwork, so yes you can manipulate a photo and add it to a still life or scene you're creating.
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