Some customers have limits as to what file formats they are able to receive. It’s recommended to send PNG and JPEG images to your customers as these are the most commonly accepted file formats. If this is an issue you're running into frequently, the best thing to check first is the image's file format!
If it is a PNG or JPEG image and it still won't send, then there may be something else going on. Please reach out to us through the Help button in the Settings and Profile dropdown menus. We'll get back to you as soon as possible to determine the cause!