Question: do i need to make everything complete? Or just the parts? 

Explanation: i am developing the whole 2d art of a platform game in a traditional way, drawing in paper and so one. But, this will be my first game and i am not programmer. So i'll make all the art and then i'll search for a partner programmer. Well, the point is, i don't know if i have to make just a large image, the whole stage, or just the individual parts such as the background, grounds and so one, and the programmer will join them at the moment of making it in unity, for example. Many thanks. I'm sure it is a stupid question for you, but it is a real trouble for me, because it will configurates the making differentially if one or another. Thank you!!

You can bring on the programmer when ever you like. You don’t even have to have complete art to develop the game. You can just make sloppy place holder art to use in development and then switch it out for the real art in the end. Though I say it’s better to get the programmer involved sooner. Because maybe he will need certain size images, or images at certain resolutions, or maybe images in sprite sheets as opposed to one big image or individual images, etc...