Goto to the sub Forum that you want to create a thread in
Click the three dots near the top right
Select New Topic
From here you can type in a title to the thread and a description. Now to then add an image to the thread select the image button next to the camera button. This should then take you to your photos on your device so you can select the photo you wish to add.
For the photo size choose Medium
The photo is then added to your thread and you can click Create to make the thread
Replying to a thread
Goto the thread that you want to reply to
If you just want to write a text reply do so in the window that says Write a reply... and click send. If you want to write a more detailed reply or add images click on the plus
From here you can add an image the same way as shown previously. Just click Reply when finished.