If you use a picture in an In App Message, make sure your image is 800 pixels wide and 200 pixels tall at the most. Any taller will make a scrollbar appear and users will have to vertically scroll to see the whole message. Any wider will proportionately scale down the image to fit the box. Be sure to adjust the size for text when cropping or resizing images.
Please note that this resolution might not be a best fit for every engagement type. For example, Guided Tours have a smaller window size, and it would be best to use a smaller resolution (500x150) image. As a best practice, please make sure to test the engagement in Preview mode before setting it to Active.