What you are doing, by clicking "New Post" at the upper right is creating a completely new thread.
If you wish to reply to an existing individual post within a thread, at the bottom right of that post, you will see a "Reply" button. If you click on "Reply" on a post within a thread, it will open a white box for you to type your reply. If you are at the last post in the thread, this box will already be displayed. This box is for entering "quick" replies which have no need of formatting or images.
If you wish to add images or use different typefaces (eg. Bold, Italic, different fonts, different colours), then click on "Use rich formatting" to use the enhanced editor to enter your reply (much like the editor we used preciously before the upgrade).