Hi I am nitin I am an engineer and lead android and backend developer. I think I can do this project in 5-7 days given the information you have provided.
1) We will decide wireframes first. I think there will be login/ register screen, camera screen, select watermark screen etc. It would be awesome if you already have wireframes and PSD.
2) I think for this app we don't need backend until unless you want the user to save the image on cloud. Saving images in local phone storage should be easy.
Contact me if you want to discuss this project in detail.