When placing images inside the Tip all you need is the <img> tag with its src and dimensions. As mentioned earlier in the Help Files when using HTML for the Tip it is best that you create a workpage. This workpage should be saved in the same folder as the page where the Tip will be used so that any image paths etc.. will be correct.
In the workpage insert the image you want in the Tip. Then go to code view and copy the <img> tags and everything inside of them. The code will look something like this..
<img src="images/Flower3.jpg" width="100" height="75">
Then in your original page where the Tip will be used select the link you are applying the Tip to and launch the ToolTips FX interface. Paste the code you copied into the "ToolTip" text field. Set the dimensions of the ToolTip ( width and height ) to the same as the image.
The next example uses a larger image but the code is basically the same other than the src and dimensions.
<img src="images/Thumb3.jpg" width="250" height="150">
There really isn't a limit to the size of the image you can use in the Tips but if the image is large you might consider preloading it so that there will not be a delay in displaying the image.
You can preload the image(s) using the Preload Images behavior located in the Dreamweaver Behaviors panel. This behavior is applied to the <body> tag onLoad. Select the <body> tag in the "Tag Selector" at the bottom left of the Dreamweaver window. Then go to the Behaviors Panel and select "Preload Images". Browse to your image and press OK. The image will now preload.