Below the message box you write your message, are three small boxes saying "choose file". If you click on one of those, it should take you to where photos are stored on your device. Click the photo you wish to use, and it should then appear. Depends on which device you are using!
When you open a thread scroll down you will see ‘choose a file* click on one and that will take you to either your camera to take a pic, your gallery to choose one you already have or browse to choose from the internet. Once you click on the picture you want it will upload it and you can either preview your post or simply post it. 😄