Insert Image button grayed out

I inserted a number of screenshots in several documents last week. When I came to add one more screenshot to a table cell in one of the documents, the 'insert image' button was grayed out.

I tried clearing the browser cache, logging off etc., but the issue remained. The problem exists in IE, Chrome, and Firefox. I have this problem with only one of the documents that I created (Part 2: Use SAP Web IDE to Create a Fiori App and to Search for UI Texts), which suggests something in this specific document is at fault, but I thought the functions in the toolbar would be independent of whatever I put in the document.

Any ideas on what the problem might be and how to solve it?

Thanks!

Ian