After uploading a new set of images you may notice that some of the images are represented by a placeholder that says "Image Being Processed." This placeholder represents the actual image until it makes its way through the processing queue in our system.
Once the image has finished processing, the placeholder image is replaced with the actual image. Depending on the number of images ahead of yours in the processing queue, the time it takes to process your images can take anywhere from a few seconds to (in cases of a very heavy processing load) 45 minutes or more.
See the following article for more details about this: How long will it take for my images to process after being uploaded?