Check your image file type - for app graphics, all images must be saved as .png at 72dpi.
Floor plans, speaker or attendee photos or exhibitor logos, can be either .jpg or .png.
Sometimes, images have an incorrect extension or are simply renamed, which does not change the image file type.
Images must be hosted on a secure site (https://) or some operating systems may not display them.
Upload your image in FotoForensics to find out if you may be actually dealing with a .gif file that was renamed to have a .png extension.
Open the image in your favorite image editor and re-save it as a .png file (or .jpg file for speaker or attendee photos or exhibitor logos). Saving an file by choosing the correct format from an image editing tool converts the image to the new file type.