There are 4 options to add a picture into a note:

1. Direct image link from other website

! [ image title] [ image url]
![Google Favicon][https://google.com/favicon.ico]
 
Google Favicon
 

2. Link to the uploaded image

You can link to it in an image block using link building rules:
![title][37], for the image with ID=37
![title][image.png], for the image in the subnotes of the current note
![title][/images/image.png], if the image is in the images folder of the current notebook
![title][//Notebook 1/images/image.png], if image is in the images folder of the notebook named "Notebook 1"
 

3. Upload an image

Use "Upload Files" menu item:
add-file-menu.png
 
This way images will be uploaded to the server, then their links will be added to the current note as ID-based links, so you can safely move and rename images.
 

4. Paste image from clipboard

Not all browsers support this feature, therefore you should test it by yourself: just create a new note, make a screenshot, then paste an image into the note. If it works, the image will be uploaded and pasted into the note.