Why is the image modified in the assets folder not getting updated on customer portal in production?
Customer Portal pages, Images
This can happen if the image you are trying to update is under the themes folder at "euf/assets/themes/" because this folder is created for the production version of customer portal during the deployment process. So if you make a change to images under the themes folder please use the Deployment Process for the change to be reflected on customer portal in production.
However, if you modified an image file under euf/assets/images those changes are reflected immediately and do not need to go through the normal deployment process.