While emotional responses to images are highly influenced by image content, the effects of image attributes on emotion is less established. Understanding and modelling of these effects will enable us to design imaging devices and software for better user experience. In this paper, the relationship between the image characteristics and the emotional responses was investigated. Eight generic images as well as two personal photos were used as the experimental stimuli, manipulated in terms of image colourfulness and lightness contrast. Two experimental techniques, including psychophysical and psycho-physiological methods, were used to measure the emotional responses. In the psychophysical assessment, each observer rated the images in terms of pleasant/unpleasant, exciting/calming, natural/unnatural, appealing/unappealing and like/dislike. The experimental results show that image colourfulness and lightness contrast have a consistent influence on the observers' responses. The observer responses were found to be significantly affected by the image subject, and in particular by whether or not there is personal relationship of the observer to the image. In the psycho-physiological assessment, each observer's skin conductance and the heart rate variation were recorded, showing significant differences between generic images and personal photos.