Best practice for uploading images to a web server

My target from this question is to know what is the best approach for uploading image(s) to a server? and why?

By passing into several web forms contain upload image for some reason (profile, post...), I noticed that by selecting the image, is directly uploaded to the server before submitting the form! Why isn't uploaded while the user is submitting? Where is the image uploaded and what if the user discarded the post? Let suppose the images are uploaded to temp directory after submitting, the images should moves to non temp directory, so why the developers are mostly using this approach what is the target? it's for the security wise? too many questions...

Any one can declare or provide me an useful article that answer or explain the case above?

Thanks.