When you're in the dialog box for a post, hit the button that says "Manage attachments."

Make sure the pictures you want to use are saved as .jpg files or one of the other options on the list.

Then click one of the "Choose File" buttons, and show it where the file you want to use is - select it.

After you've done that for as many files as you want to attach, you NEED to hit the button on the right side of the dialog box that says "Upload." Until you do that, nothing is really attached. Once it says your files ARE uploaded, you can continue and post your post!