Let's say that the max size your visitor should be able to see is 800 pixels. Then go to coppermine's config and set
- "Create intermediate pictures" to "no"
This way, the full-size image will be used on the displayimage screen as well - "Max width or height of an intermediate picture/video" to "800"
This will take care of the resolution you will see of the embedded pic on the displayimage screen - "Max width or height for uploaded pictures/videos (pixels)" to "800"
This would result in an error message if your users tried to upload images with a bigger resolution if you disabled the next option "Auto resize images that are larger than max width or height" to "No". However, if you set "Auto resize images that are larger than max width or height" to yes, all uploads will be resized to what you have set in "Max width or height for uploaded pictures/videos (pixels)" - "Auto resize images that are larger than max width or height" to "Yes:Everyone"
As suggested above, you need to set this to "Yes" to make sure that users don't get an error message when uploading bigger pics than they're suppossed to. Coppermine will then resize the images on the server if needed
After applying those settings and testing them by uploading some new pics, you need to apply those settings for the existing pics as well: go to admin tools in coppermine's admin menu and choose "Update thumbs and/or resized photos" with "Both thumbnails and resized pictures" and loop through all files in your database.
Above mentioned settings are the most efficient ones, as they safe as much file space as possible and make it impossible for visitors to get the full-sized images in larger resolutions. Just keeping everything as it is right now and just disabling the link from intermediate (embedded) image to the full-size pop-up is technically possible as well but can easily be circumvented by visitors who know their way around.