Summary: On your project's Confirm Location screen, tap "Import" on the upper edge of your screen. Select the blueprint image you'd like to measure from, adjust the image and tap "Snap and Start Drawing."
When importing an image, make sure the image is in .JPEG format to avoid errors.
Watch the video below to see the process:
Note: To set the scale for imported imagery, you'll need one, verified, on-site measurement.