To create a new empty animation, click New from the File menu
To add an existing image to the current animation, click Add Frame from the Project menu and select the image you wish to add. Alternatively, just drag and drop images from File Explorer.
Select the frame who's duration you wish to change, then, in the Frame tab, enter a value in milliseconds into the Duration field.
On the Animation tab, check the Looping option to have the animation loop, then check either Infinite to have the animation loop non stop, or enter the number of repeats in the Repeat Count field.