it's too hard to show the picture from the book without giving away what the creature is,
So describe it in words. The same way you (presumably) describe everything else in words when you're a DM. I mean, you don't require pictures of every location or NPC the PCs encounter*, do you? Being able to give evocative descriptions purely in words is possibly the single most indispensible trait for a DM to have.
If there's no 'official' description (or picture) of what something looks like, then you can describe it any way you want. You literally cannot be wrong, so stop worrying and let your imagination run riot.
* - Although using portraits for recurring NPCs is actually quite useful, since it saves you from having to re-describe them to players who've forgotten how you initially presented them (or aren't quite sure which NPC this is, because they just cannot remember names).