What users can and cannot do, including which icons are greyed out, depends on their permissions. Here is a summary.
Asset Bank "View" permission required
The user will see all files in the connector that they have view permission for in Asset Bank.
Asset Bank "Download without approval" permission required
The user will only be able to place files for which they have download without approval for in Asset Bank.
Downloads are also logged against the user utilising the connector to place a file.
Asset Bank "Edit without approval" permission required
If the user has edit without approval permission on a file then they can upload new versions of files that they have changed (e.g. after making adjustments in Photoshop for example). This will appear in Asset Bank as a new version, so the original is still possible to get to (if you have permission to view older versions).
If you visit the version history you will also see the name of the user who made the update:
Uploading new files
Asset Bank "Upload without approval" and "Edit without approval" permission required
Users with both Upload and Edit Without Approval permission on certain folders can upload new files directly to these. Please note, this will bypass any default values for metadata and will not force the user to enter any metadata other than filename.