The Ripl app will capture the first frame of the video, or the "thumbnail," before you leave the "Customize" step. The design will capture the current frame at the time you tap the "next arrow" on the "customize layout" step.
If you want the "first frame" to be the first photo, then navigate within the design to that photo using the turquoise arrows before you tap the ">" in the upper right corner of "Customize" step.
It's important to note that each social network where you post the video may select the thumbnail of the video differently. On Instagram you have the ability to select a different "cover" for the video in the same place where you select your "filter." On Facebook we've noticed they often default to a random frame in the video, spread across 10 different possible frames. If you "edit" the post (in the upper right corner) you'll have the ability to select a different thumbnail.