The insert image button opens the window shown below.

You simply enter a URL to the image you want to link to and enter a image description, this is then displayed as an alternative text descripton of the image on the page.
Field descriptions:
| Image URL |
URL/path to the image. |
| Image description |
Alternative description of image contents. |
| Dimentions |
Image width/height. |
| Alignment |
Image alignment, useful when wrapping text around images. |
| Border |
Border thickness. |
| VSpace |
Vertical space, useful when wrapping text around images. |
| HSpace |
Horizontal space, useful when wrapping text around images. |
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