I would normally contact the seller first too.

But I am a bit confused as to what good a photo would do you, apart from actually showing you what she claims to have received. 

If the woman wanted to cheat, all she would need to do is remove half of whatever it is then take a photo.

You can't actually take a photo of something that is missing.

However, is there any possibility for confusion with your order? For example, could someone think they were getting eg 2 of something when they are only getting one? The sorts of ads I am thinking about are ones I have seen where it says eg 8 pieces but when you read the ad properly it is actually 4 items with 4 screws for them, something like that.

So I think you need to reply to this woman and outline what was in the packet and ask exactly what she received (if that hasn't been established already).

Then if it appears there is part missing, you could suggest that you did pack the entire item but perhaps there has been damage to the envelope in the post.

 After that, if you are sure you packed it all,  I'd be inclined to wait till she opened a claim.

I think some buyers go under the radar by getting refunds or partial refunds without ever opening proper ebay claims, so there is no record.

Yes I would contact the seller first.

As a seller I would wait for the buyer to open a case for Item Not As Described.