* A fair use image has to meet all of the fair use criteria. If a free replacement could be created then a fair use image is not allowed (fair use criterion #1).  
 * Multiple images are not allowed if one is sufficient (criterion #3) 
 * Fair use images may never be included as part of a photo montage, as their status as being "fair use" depends on their proper use in the context of an article (as part of criticism or analysis) (image use policy). 
 * A fair use image must identify the subject of the article or illustrate certain parts of the text (criterion #8) However, if a free image could be created then identification is not permitted (criterion #1). 

 Unfortunately the subject of this article is generic and not specific (an article on test images in general can be compared to an article on oil paintings or on coats of arms). The possibility that a fair use image could be replaced by an image available under a free license cannot be ruled out. 

 I would also like to point out that articles on similar topics like Color calibration, Color management, Linux color management and CMYK color model are illustrated only using free images. The same is true for the Cornell Box, the Utah teapot and the Stanford Bunny. However, removing any images should not detract from the quality of the article text in any article.