Craigslist.
Some say Craigslist is a dying concept. (another topic)
Some say social networking is a waste of time.
Some say and some do.
This is for those that do and those that can handle a little HTML.
Step 1. Host your GIF file. I recommend keeping the animated GIF as small as possible. The one I used in this blog is 8.7 MB. The One in my Craigslist post is only 1.3MB. Performance is always a consideration when you include one if these files. I make the assumption that my audience will be using broadband of some kind. If they're using dialup, then they're probably not a likely customer either.
You have to find a place to store your .GIF file that is publicly accessible via an <IMG> tag and allows GIF animations. There are many free web hosting options. I know that Google Sites (not blogger) works. You can get a free account by signing up with Google (free). You can start at http://sites.google.com. Since I'm a proponent of Google services, I'll leave it at that.
Step 2. Create your Ad. This isn't really a tutorial about creating ads. Craigslist changes what they allow in design elements whenever they want to. A lot of agents use 3rd party software to create postings. If you're using that, you have to find your way through the software or your completed post to insert your animated GIF file.
Step 3. Embed your .gif file like this: *note, this is the HTML you will type. <img src="http://davekeys.com/images/mauipics.gif">
Anatomy of the HTML:
Orange: This is always the same. You embed an image, including the animated GIF file with the IMG tag.
Blue: This is a reference to your GIF file on the server where it lives. If you use a subdirectory you include it in your location. You can usually copy the link to an image file from your browser.
My Google Sites link looks like this: http://4490269834147606439-a-1802744773732722657-s-sites.googlegroups.com/site/davekeys/gif-test/mauipics.gif
It works like this: