You would be offering a service, and would have no rights to usage of the photos in their original or edited form unless you specified that term in a contract with the owner of the image. You would be selling your time, not taking ownership of the source material.... if you want rights to the image, pre or post edit, you would have to purchase the image from them, and get permission to distribute an altered version of that image. Even if you do purchase an image, it is common to have restrictions placed on the distribution of the image.
Let's say I hire you to edit an image for me... It's a high quality image and needs some post processing to make it more vibrant and give it more contrast... perhaps some element of the image needs to be removed to clean the image up.... So you edit the image and you do a lousy job and it looks terrible (to me... I'm fussy and don't like how you've manipulated the image). Then you post that image on a website and give photo credit to me, without mentioning the specific process that you have used to edit the original, and you don't show the original for comparative purposes... What do you think happens next if you haven't purchased exclusive rights to that image and all usage there of?....