Login to InVision https://projects.invisionapp.com/d/login
Click on the project you are adding to.
Click the plus sign to create a new preview
Drag and drop the screenshot.
You can position the screenshot so that it's at the top. When you drag the screenshot onto the page, the Drag & Drop overlay will disappear and you will see a large curser. Position where you want to put the screenshot and drop.
Click the "Share" link
Copy URL and paste into basecamp in the appropriate to-do.
If you are working through the "Most Engaged emails" steps, you will need to continue on to the final steps.