You can add image to the topic by two ways:
1. Click "Picture" button on the topic editor toolbar and select image file name. This image will be "local" topic image.
2. First add image to the "Images" node. This will make your image global or "library" and you will be able to use it in multiple topics. When image is in the "library" place a pseudo-tag like
where you want this image to be. "yourimage.ext" is the name of your "library" image.
There's an alternate way to insert a Library Image. Place cursor into the topic editor where you need image to be placed. Right-click, select "Insert Library Image" and select library image name. Image will be shown as it is with header like "Lib img: <image name>". This is more WYSIWYG-style but image occupies more space in editor than just a placeholder.