You can use Actions (conditional logic for your giving page) to thank donors for their generous gifts. In this example, I'm going to surprise donors who give more than $1000 with a fun gif.
First, add the gif and/or message to your giving page.
In this example, I'm using a paragraph text field. You could also use a picture field.
Select your Actions tab and create a new action.
Set up the action to SHOW the field when the amount is greater than $999.99
Save the action. You can test to make sure it's working in preview mode.