In federal courts, the prosecutor must prove that you possessed or received child pornography. Receipt of one image of child pornography carries a potential five-year mandatory minimum. The elements of receipt are very similar to possession and there have been many articles written about how it is a matter of prosecutorial discretion regarding a charge of receipt as opposed to possession of child pornography.
A receipt of child pornography charge can arise out of downloading, trading, or being just an image. You are receiving the image from somebody or from somewhere. Possession is the act of having the image at issue.