The message "Image(s) with instance cannot be deleted" indicates the image or DICOM is being used in either a case script and/or a templated item.
Remove the image from the script and templated item and return to the Image tab to delete the image. See below for more information on scripts and templated items.
Case scripts are located on the Script tab of the case. Locate either the image or templated item in the Case Presentation Script and select the item. Use the [Delete] button to remove the item from the script. If the item removed was a templated item, the image must also be removed from the templated item before it can be deleted from the case.
Templated items are located in the Templated Items tab of the case. Check out the case then find the templated item (red arrow). Select the templated item. Select the image to delete and then delete the image. Note: Different templated items may display the image and [Delete] buttons slightly differently.