The visibility of images displayed on mobile devices can vary significantly according to the lighting conditions. Generally, the brightness of a mobile device is adjusted to enhance the visibility according to the intensity of the illumination. However, different brightness settings can be more suitable for different types of content, even under the same illumination. Accordingly, this article presents a method for adjusting the device brightness according to the features of the displayed app image. First, two subjective tests are performed under various lighting conditions to select the features related to the visibility of several app screenshots and to select a satisfactory range of device brightness for the same app screenshots. The relationship between the app image features and the satisfactory brightness levels is then analyzed. Thereafter, the images are categorized using two features: the average luminance and the advancing color ratio related to satisfactory brightness. The optimal device brightness for each category is then selected based on the maximum frequency of satisfactory device brightness. Experimental results show that the categorized app images with the optimal device brightness produce a high satisfaction ratio under various lighting conditions.