Well, the title describes what I want: usually users post words as a hint, but in my case I have an image ... is it possible to spoiler-tag it ?
Yeah, you can.
Put the image markup after the spoiler markup, like this:
Image by Diliff.
Source code for that is:
>! ![fish - this is the alt text] : http://i.stack.imgur.com/xbACz.jpg