Hi Sally, if you're seeing an error, please note that the original image and the replacement image need to be the same file type (for instance, if the original is a JPG file, the replacement must also be). If you're able to upload the replacement successfully (you'll know it was successful if you see a window that says "Image Uploaded" and has a "Done" button), you should be good to go after clicking "Done". If you're still seeing the old image at the URL, you may need to clear your browser's cache. If clearing your cache doesn't work, please contact Marketo Support and we'll be happy to look into the issue for you!