The most common reason that an image is missing when you preview a template in Core iQ is because of a bad link to the image file. A quick way to fix this is to grab the URL for the image again using the Copy URL button in the Template Artwork tab. Then, find the image in your template builder and replace the URL by double clicking the image and pasting in the URL from the Template Artwork area. Save the template again, then use the Desktop Preview to make sure that the image is correctly rendering. You can even send yourself a proof using the Proofs & Review button to ensure that the image is now showing up correctly in a fully-sent email.
The second most common reason for images to not display in Core iQ (if the URL is correct) is that your connection to our cloud-based image hosting service, Cloudinary, is being blocked by your firewall. You an easily figure this out by sending a proof of your email to your personal device and viewing that email while on your mobile network (not your institution's WiFi). If the images display correctly on your personal device network, then they are simply being blocked by your institution's firewall.