Over the button [Shopdaten] you reach the window with the extensive settings, which determine the appearance of your Shop:

If the article pictures should be adapted automatically, then this can be selected here.
Indicate the maximum size of the article pictures. If a picture is bigger, then it will be made smaller automatically (the proportions are naturally maintained). If you liked to select only the width or the height, you can set simply the other dimension to 0.
Hereby the desired quality of the article pictures can be selected. Please consider that a high quality result a big pictures and a bottom quality result a smaller picture (jpg compression).