You can attach an image by clicking the paper-clip icon on the toolbar when posting. Click "Choose File" then "Upload" or paste in the url to the image if it is already hosted somewhere. After you do that, close that window and click the little down arrow next to the paper clip to insert the image into your post.
Keep in mind that uploading the pic will use plantedtank.net's photo hosting, which I believe has limitations (plus it costs the site money and there are many free photo hosting sites.) I recommend imgur.com, you don't even have to sign up for that one if you don't want to, just upload pics and then you can link to them.