• Option 1: Drag your image in the content or sidebar, double-click on it or go to RW > View > Show Media Editor: now the Media Editor shows up. Only add the word flexible in the input field of the Alt Tag in the Media Editor:
To see it in action, open the content on a desktop browser and slowly make the browser narrower and wider... Or have a look on your iPhone or iPad. This is another example in the 3-Columns stack.
• Option 2: When your image is on the server (here's a nice tutorial about image warehousing for RapidWeaver projects) simple copy&paste this in the content or sidebar as plain text:
So, with alt="flexible" behind the image link. Replace the name of the image with the name your own image.
<img src="http://www.website.com/images/image1.jpg" alt="flexible">
Test a working example in your website and have a look in preview:
<img src="http://www.henkvrieselaar.com/tulips/tulip3.jpg" alt="flexible">