The only file extensions that are supported in the apps are .png and .jpg for speaker photos, exhibitor icons, or floor plan files. Sometimes, image files are merely renamed but not correctly re-saved and a file called "image.png" is actually a .gif file format. For example, if you rename a .doc file into .pdf file, an actual PDF file was not created. You may even see the image appear in the CMS, but only because the browsers are very forgiving, unlike native mobile operating systems.
Images must be hosted on a secure site (https://) or some operating systems may not display them.
To test the true file type of an image, upload it to https://fotoforensics.com/ and verify it is what it's extension says it is. If the extension does not match the actual file type, open the file in a graphics program and select File > Save as > and choose either .png or .jpg to re-save the file correctly.