That depends on the client, of course.

A thumbnail is generated when you upload the image, but the client you use might show you (by default or by configuration) the big image directly.

Anyway, not that it's a problem, it's just that I saw that the full image was 3000*4000, and it seemed a bit overkill xD

Most people will probably only see the thumbnail, so as far as server load is concerned, it's probably fine =)