To create a dynamic image, you need to make a variable that you'll use to replace the image element src with. In the right panel, there is a section called "Variables". Create your variable by pressing the "Plus" button.
A dialog will open where you can provide the name of the variable and a default value. Note that the default value is optional. The default value will be used if you do not provide any content for that variable when you use the template. In our case, the default value for your variable could be a placeholder image URL.
Now to use the variable, you can create a new image element. You can upload a placeholder image for now, or close the dialog to keep the blank element. Select your new image and you will see two new fields under the variables section where you can select which variable to assign to this image element. Select your newly created variable as "Replacement key".
When using the template, you can pass a new image URL under your variable name. Your new image will be fetched and replaced on-the-fly!