To annotate an image with bounding boxes, you need to access an Image View. There, you can click on the eye icon, next to ANNOTATIONS, on top of the navigation bar: this will either hide or show the image annotations.
To create a new bounding box, click on Add a box, select the concept for this new box, and draw it on the image. There are 2 drawing modes, you can switch between them by pressing the SHIFT key:
Classic mode: click on the position of the first corner of the box, drag the mouse pointer to a second corner. By releasing the mouse, this will create a box.
Extreme points: click on the 4 extremities of your object (top, bottom, left, right) without it necessarily being the corners. The box will be created to frame your object.
To switch from one annotation mode to another, press the Shift key on your keyboard.
You can remove a box directly from the navigation bar, by clicking on the trash icon.
Keyboard shortcuts are available to make the annotation faster and more convenient. You can get the shortcuts list by clicking on the "?" icon on the bottom right: