Here are the cards from Chapters ($12.99) we have been using. There are different themes of cards. We have the 'Mystery in the Forest'. Oliver loves animals so many of stories revolve around the squirrel, fox, pig or chicken. And of course the treasure.
I also found two apps that essential do the same thing. One is Story Wheel. What's cool about this apps is that it tells back your child's story in their voice. And you get to spin the wheel kind of like the Price Is Right.
The other is Story Dice. Instead of cards you get random images on the dice as prompts.
I came across as a website called Kitchen Counter Chronicles that has a free printable for you to put prompts in a jar. Here is the link: Storytelling Prompts for Kids.
Blocks can also be a great tool as most have pictures on them.
So many great ways to get your child's imagination going. May your imagination take you to some fun places!