The question is a bit unclear. Do you mean to change the "IT Service Intelligence" label that is shown in to top right corner? If yes, then you need to create a .png file with whatever content you want to display and save it under the /etc/apps/itsi/static as appLogo.png (size must be 160 x 40 pixels) and as appLogo_2x.png (size 320 x 80 pixels). These are maximum sizes. Create png files with transparent background to integrate with the existing colors. See Configure App Properties for additional info and other .png files that you can configure for an app.
If you want to add an image to one of the dashboards you need to create a corresponding background image and replace the existing one (or create a new dashboard):