Dynamic countdown timers
Another great application for dynamic images is countdown timers. Here you will be passing in the Dynamic URL the date of an event; like the launch of a new product or the date and time a promotion expires. The system on the other side will automatically generate a GIF animation that will look like a countdown timer, signaling the time remaining to the event.
A placeholder image can be used in the URL while you design the message (or page), so you know exactly how the end result will look like.
Don't want to use a 3rd party solution? For an HTML based countdown timer, see this guide from Email Design Workshop.