There are several ways to add an image, the simplest is to use the forum's built in file upload feature.
Simply navigate to the thread you wish to add images to and click 'Upload a File'.
A file selector box will pop up.
Two examples are shown below.
Find the image you want to add and click the affirmative option (open, select, yes, upload etc). You can select and upload multiple files at once on supported devices.
Your image will upload, usually instantly.
It will show up under the message box as a small thumbnail, if you were to click 'Post Reply' at this point, the image will show up below your post as an 'attached file'. If however you want it to show in the post itself you have two options, you can insert the full sized image by clicking 'Full Image' next to the thumnail, or you can insert a small thumbnail by clicking 'Thumbnail'.
In this example I've clicked 'full image'.
You can then write the rest of your message. If you click 'Post Reply' the image will display with the rest of your text.