You can't create your own tokens. There has to be code within the software to support it.
If you're just trying to place an image, it should be easy enough just using:
<img src="path-to-your-image-file" border=0>
* Of course, you need to enter the actual path to your image file for the blue text above. An easy way to do this is to find the image using Windows Explorer, then right-click on the image while holding down the Shift key, then select Copy as path. Then just insert the copied path.
You would then just have to change the path-to-your-image-file each time you want to change the image. In order to size the image appropriately, the <img> element takes height and width attributes:
<img src="path-to-your-image-file" height=300 width=200 border=0>
If you don't specify height or width, the image will appear at full size. If you specify both height and width, the image will be scaled to that size (possibly changing the image proportions). If you only specify one of them (height OR width), the image will be proportionally scaled to that size.