Hello Simon!
If the image is a JPEG, PNG, or GIF, there may be an issue with the image itself. The file may be corrupted, it may be saved as the wrong file extension, or a host of other issues. Best option would be to open and resave the image to see if that sorts out any issues. Below are a few things to try:
If you have a photo editing application like Photoshop, that would be your best option. Open it up and “Save for Web” as a High Quality JPEG.
If you don’t have Photoshop but have a Windows PC, you can try opening the image in Microsoft Paint and saving it out as a JPEG or PNG. With the image open, click File > Save As > JPEG (or PNG).
If it’s not a very large or high resolution image, double-click it to open, view it full size on your screen, and take a screenshot of it (shift+cmd+4 on Mac, or use the Snipping Tool application on Windows). Then save that out as a JPEG or PNG.
If the image won’t open in any applications, that means it’s likely damaged. The only real option then is to go back to the original image on your camera or storage device and try re-downloading it.
Hope this helps! Let me know if any of these options work out.