Click on this icon from toolbar where you type post.
This will pop up, you will need to hit ( choose file ) from photo library.
Then select your photo, and choose the size if you want. Small loads faster. Like below.
Hit done after selecting your photo, and then to the right is a button that says ( Add file from your PC)
click on it and it will add photo below, if want more repeat second step through this one. When have all photos you want hit ( All done ) tab and it will insert photo into your post. If you preview post you will see the actual photos.