Adobe Spark is a new platform by Adobe that is available at http://spark.adobe.com or on your ios or android device. The App connects to the Adobe Spark community and all creatives are available in your Adobe Gallery.
There are a few limitations. Adding multiple images is not possible currently, but I am sure that Adobe will add this feature in the near future.
Images can be resized to any of the standard platform sizes with the click of a button. Custom sizes are not yet available.