Let's say your resolution is WxH. So you are creating a window with dimensions WxH and you are creating a backbuffer with size WxH. Then you are drawing the backbuffer to the client rect of the window. The catch is that the client area of the window is usually smaller than the window itself - the dimensions of the window also include the title bar and all the borders. This way you are trying to draw an image onto an area that can't contain it without distorting it first.
If that's the case you want to resize the window and make it larger so that it can contain client area that has the exact same size of the backbuffer. To do it you call the AdjustWindowRect system function.
Judging from your name and the course you are asking about, I can tell you are most likely studying at FMI, Sofia University. If that's the case, I can assure you it's a complete waste of time if you are trying to learn something about computer graphics. I enrolled the course several years ago, because there was no one who could tell me that the name was quite inappropriate, given the things that one can learn from it - defining simple shapes and objects with certain properties(like texture and size) and applying simple operations to them. Keep in mind all of these things are not done in a general way that can help you in other situations. It's strongly Elica specific and Elica is a very high-level language/framework, so it certainly will not help you grasp the concepts of 3D programming. Ah, one more thing - Elica is slow, so it's very, very poor choice when it comes to game development.
With that made clear, it still was an interesting experience, especially if you take into account the lack of gamedev-related courses at the Faculty