When you record a Ripl video, Ripl captures 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 ">" on the "Customize" step to proceed to the "Caption Your Post" step. If you want the "thumbnail" to be the first photo, navigate within the design to that photo using the turquoise arrows on the design, before tapping the ">" in the upper right corner of "Customize" step.
Each social network where you post the video may select the thumbnail video differently. Instagram allows you to select a different "cover" for the video in the same place you select your "filter." Facebook will 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.
When sharing to Instagram, you may want to select the "Cover" option within the Instagram app to choose the preview for your video manually.