People with the autism spectrum have special needs when it comes to using computers. Therefore designers of graphical interfaces dedicated to such people should bear in mind the limitations faced by them. In this paper, the system, which supports designers by automatically evaluating the accessibility of graphical interfaces, is proposed. Image processing methods are used to extract features, on which the introduced fit scale allowing for evaluation of interface adaptation to special preferences is based. The presented approach facilitates the creation of interfaces intelligible for autistic people and raises awareness about their needs. The fit scale values were calculated for several game interfaces and the obtained results have been compared with a visual analysis performed by an expert working with autistic people.