Exposure will handle resizing of photos up to 10MB in size but for the quickest and best results we suggest photos of 3000px along the longest edge at 72DPI. This will cover any screen resolution. Accepted file formats are .JPG, PNG and .GIF. 

Our system will automatically resize your photos based on the screen size that your story is opened on. 

If you have questions about photo dimensions then please let us know!