From Test-Scratch-Wiki
Import method
To import a sprite, click the icon made to look like Giga at the very left of the bar and choose a sprite from the list.
Paint method
Another option is to paint a sprite, by clicking the second furthest left button which is an image of a paintbrush. This opens up the Paint Editor.
Import from Computer Method
If you click the button which is second furthest right on the bar it will allow one to select a file to upload from one's computer.
Camera Method
Clicking the camera will open up a screen displaying the view from your computer's webcam. There are options to "Save" and "Close". The picture taken from the webcam then becomes a new sprite.