First thing to understand is that we do not host email images in our servers because of cost and maintenance. This means that the image has to exist elsewhere online in order for us to pull the data from and use. Either the image is already from Google, or you have posted it somewhere. If that is the case then you can just right click and "copy image address" and then paste that address into the picture field in CW.
If the image is not already online somewhere, and you are uploading from your computer, then you will need to upload it online before inserting into CW. There are two really useful websites that exists for this reason and that are free to use: imgbb.com (recommended), you can also user imgur.com. If you are using imgbb, click upload in the top right corner, upload the photo in question, be sure it is set to not auto delete, then you will click on the image and it will open in a new window. From there you will simply need to right click on the photo and "copy image address" like you see above. Then you can paste right into here: