The reason why you will synchronise images is because if you have FTP the images into a folder and you will need to synchronise the images to select them in Image Manager.

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Go to: Dashboard > Publishing > Image Manager
 
Click on Synchronise Images



Click "OK"




Select the Images and click on "Synchronise"
 
Note: If there is a large number of images to synchronise it would be best to synchronise the images in groups of 10 for example so it does not time out.

Note: The folder needs to be active so the synchonisation of images will work.