From FCKeditor Docs

Sometimes a user can try to upload a file which isn't an image file but appears as if it would be a one. Example: You have a text file called document.jpeg and you try to upload it. If you enable the function by setting it to true:

config.secureImageUploads = true

the program will check the dimensions of the file. If they are zero, then the file isn't an image and it will be rejected be the CKFinder.