You can insert an image into the text using the 
Insert/edit image button in the WYSIWYG editor toolbar. The Image properties dialog opens:
You can either insert the URL address of the image if you know it or you can click Browse Server and choose the image in the content tree:
If the image hasn't been uploaded yet, you can click Upload file in the upper right corner and choose the file to be uploaded from your local computer:
Click Save to upload the image and then click Page image in the image preview:
Now you can see the preview of the image in the Image Properties window and change its alignment, size or add alternative (ALT) text for better usability.
Click OK to insert the image into the text:
Click Save to save the changes.