Been asked this a few times and have finally got a proper environment setup to answer this specially with an iPhone. Using this thread to answer and test...
1) Click on the icon in the reply or new post button that looks like a tiny painting of a tiny tree....
2) Select the "From Computer" Tab
And find the file you want to upload.
If all goes well, it should display as an inlined image in the post as shown above.
Next post should have some screenshots from an IOS device.