How do I create a welcome screen with the thumbnail of the agreement

2) How do I create a welcome screen with the thumbnail of the agreement before going to the actual agreement?

If you want to add a "Welcome Screen' to your form, you will have a couple of options.

The easier way would be to insert a page Break into your form at the very beginning and make the first page of your form the "Welcome Page" where you can include a logo or an image of the form to be placed as the "thumbnail".

Here is an example:

Related User Guide - How-to-create-a-multi-page-form-with-jotform

Alternatively, you could change the layout of your form to a Card Form and that will automatically give you the ability to insert a "Welcome Page" to preceded your form.

Here is a screen recording to demonstrate how this can be done: