If you were convicted of violating the federal child porn statute, the court must impose no less than the minimum sentence of 5 years in federal prison and may impose up to 20 years in prison. The DOJ would not generally go after downloading a single image accidentally if the image is promptly deleted; the image you describe likely not draw the interest of federal prosecutors. However, there is always the risk of a prosecutor deciding to go for prosecution even with a single image; the details of the image and how you got possession of it would matter. If you do get prosecuted then you face an extremely serious charge and should definitely consult a good criminal defense lawyer.

