Well, I literally just uploaded the photo in the part after it says "this text will be hidden" but before the bracket. If that doesn't work, you could always hit enter twice when you use it. Hitting the enter key replaces the highlighted text, and you could just post the image, so like this. I'll show both ways
[details=Summary] here. [/detailss]
If that doesn't work you could try it this way. (btw I added the extra s on details to block it from actually performing the hide function)
IMAGE HERE. This is usually how I normally use it, it's easier to separate the summary title and the text/images you wanna hide, y'know?