Can I put a picture in the thank you page?

I've created form then it moves you to a thank you page - can I add a picture to it?

Yes, you can do that by setting up the form to show Thank you Message and then adding the image over the toolbar.

You can see here how to set up the thank you page: Setting Up the Thank You Page

Now once you in it all you have to do to add the image where you want it is to click where the image should be located at and then click on the Insert Image as mentioned above:

Once you do, there will be a new window that will open:

In the first field you enter the link to the image and under dimensions you can set its size - the original picture size will load for you, but you can also resize the image using the little resize handles when you view the image.