Thank you for the reply.

 

Updating the editor to latest version didn't solve my problem.

 

However, I did solve it by changing my code slightly from:

 

OnClientClick = "PasteHTML('<img src =../filemanager/previewfile.ashx?id=" & e.Item.DataItem("FileID") >')"

to
OnClientClick = "PasteHTML('<img src =""../filemanager/previewfile.ashx?id=" & e.Item.DataItem("FileID") & """>')"