VGFacts

When you start a new game, it plays an intro cutscene. During the intro, a frog appears to be crossing on the road. After the frog first appears, a split second image of the frog appears to creepily smiling at the camera before it hops off as normal. It’s not known whether it was done as a scare factor, or just something someone sneaked in before the game was released.

Intro video can be seen here:
https://youtu.be/OJWrNIJMegg

I'm pretty sure it's just part of the creepy imagery of the game, which focuses on Lovecraftian horror.