Do you know a good online service for creating animated banner ads ?

may this will help you https://imgflip.com/images-to-gif

The links below might be of your interest for an online animated banner tool

1. http://www.bannersnack.com/
2. http://www.web-animator.com/
3. http://www.webestools.com/banner-animated-maker-generator-gif-banners-free-online.html
4. http://www.addesigner.com/
5. http://www.animationonline.com/
6. http://www.bannermaker.be/

Thanks

It is always better to get the customised banner design service from animated banners, Html5 Banners, Google Adwords Banners.

I suggest 

www.bannersmall.com

www.hyov.com for custom Banner design or other graphic design