How to delete or replace the checkmark image on the thank you page?

Is there a way to delete or replace the checkmark image on the thank you page?

Yes!

First, you will go to the "Thank You" step within your LeadForm. Here, you'll notice the check mark in the step.

Next, you will toggle on the "Custom Code" feature within the step options.  Notice that the text and the check mark disappear.

Next, you will type in your thank you message in the Custom HTML box and click "Save Step" to save your changes.