Uploading things to a Facebook selling forum

[LaTrucha]

I want to post some things for sale on our local swapshop on Facebook but I don't know how to upload the pics. I browse for the photo, write a message and press post (as there is no button to say 'upload') and it looks like it's uploading but then does nothing.

What am I doing wrong?

I feel like the world's biggest thickie, because obviously loads of people can do it.