Image quality measures should show how some new interpolation method performs in comparison with other interpolation methods. For the case of magnified image achieved by interpolation, original image is unknown and there is no perfect way to judge the magnification quality. Common approach is to start with an original image, generate a lower resolution version of original image, and then use different interpolation methods to magnify low resolution image. After that original and magnified images are compared to evaluate difference between them. In this paper we examine how the process of achieving lower resolution image affects picture quality of interpolated images. Our results show that comparison of image interpolation methods depends on downscaling technique, image contents and quality metric. For fair comparison all these parametres need to be considered. Otherwise, results of comparison can be incorrect.