Thanks for reaching out to the Community! It sounds like Constant Contact timed out while you were working in Canva, but as long as you published the image in Canva you will be able to use it.
Insert Canva images into your campaigns (full steps)
Step 6: Create your graphic, then click the Publish button in the upper-right when you’re done.
Note: If you want to use a graphic you've already created in your Canva account, click Folders from the left-hand side bar to view all your designs.