The Change Attributes action allows you to change attributes of any element. The element may be a DIV, IMG etc. or any other DOM element.
First, either drag from the Action to the element you want to change, or select it from the popup menu. If the element does not exist at design time, but will at runtime (i.e. it's created in PHP), then you may type in the ID of the element, or another jQuery style identifier. You may then click the '+' button to add an attribute to change, many DOM elements support their own attributes, such as 'src' for IMGs. You may then type a new value for the attribute.
When the Change Attribute action runs, it will alter the attribute you have chosen to the value you typed, so in the case of an IMG, you could change the 'src' attribute to a different image to display.