My image seems to be stretched once published on my page. Why is that?
It is important to respect the required dimensions. For the banner, you will need a minimum of 876 x 200 pixels. For the profil picture (your avatar), you need a minimum of 270 x 270 pixels. By respecting those dimensions, you can be certain that your images will have the good format on your page.
If you don’t know how to change an images dimensions, you can try PicResize which allows you to easily modify your images for free! It should be noted that the site cannot make miracles; if you try to change the size of a perfectly square picture into a rectangle, you will definitely lose the image proportions and get a deformed/stretched image.