Within DNN, you try to upload images through  Image Manager, but all features are greyed out.

This issue can occur when the files on the site are out of sync with the catalog of files in the database. You can use the File Manager in DNN to correct this.  To resolve the issue, go to the File Management tool under Admin. What you will want to do here is "syncronize this folder and subfolders". You will see that option in a pull down menu next to the "refresh folder" button.

On older versions of DNN the options may be different. You may need to check the "Recursive" box next to the "Syncronize Files" button and then hit Syncronize Files.

File Manager in DNN 7.x