You cannot view this image because there is a filter for the thumbnail field. The filter determines what images display in the image browser. During the Content Type creation the admins can apply a filter to the field, which restricts the items that show up to one Content Type. So authors are only able to see items of the designated Content Type.

To change this filter, you will need to go to the Content Type of the page that you are selecting a thumbnail for.
  1. Go to Content > Select CMS.
  2. Click Content Types > Select More.
  3. Click the Content Type of the page that you are selecting a thumbnail for.
  4. Click Edit for the Fields section.
  5. Right click on the dropdown section of the Thumbnail Image field > Select Field Properties.
  6. Click on New Filter or Browse Filters next to "Choose a content filter."
  7. If you click on Browse Filters, you will got a pop-up that will let you select different filters to apply to this field. Choose the field(s) that you would like to have included in this filter. Click OK
  8. Click OK again in the field properties window.
  9. Click Save.