Content Not Permitted, two-four-seven is not allowed in the community. Remove it & try again.

What does this mean? "Content Not Permitted, two-four-seven is not allowed in the community. Remove it and try again." 

My post had text and snipping-tool images from my Excel spreadsheet - that's all. What is "two-four-seven"? 

How can I display images from my spreadsheet without triggering this error? 

And as I tried to publish this question, I got the same warning!  So I have spelled out the numerals.