"Insert Image" grayed out -- help?

So this is my first time trying Sigil, and it's fabulous so far. But for some reason "Insert Image" is grayed out for me. I've searched everywhere I can think of for an answer and can't find one -- I feel like it's probably something easy or obvious and I'm just missing it? (I searched this forum too before posting; apologies if I missed something.)

The file is an EPUB that was converted in Calibre from an HTML file. (And I love working in HTML and am usually viewing in split mode, so HTML-assuming answers are just fine.)

Thanks in advance; it feels like a weirdly simple problem to be banging my head against...