Best way for managing images of ads banners

Depends on how you build the pages (or theme parts) that ‘grab’ the images. And how you get new images on the site.

If you load via Add Media, then they will get stored in YY-MM folders (see Media Settings). That’s OK; they will be grabbed just nicely from there.

If you have a fixed file name for your banner (you manually update the same file, and your template/pages call that specific file), then you might be able just to upload the file.

No matter how you do it, make sure that you upload optimized JPG files, for faster loading in the browser.