If you double click on a sprite (or right click, "Edit animations") you open the image editor.
With in the image editor there is an Animation frames window.
Right click on the Animation frames window and choose "Import frames"
(delete the first frame since you probably won't need it)
After the frames are imported you can move them (by dragging them around), duplicate them (right click, "Duplicate") and set the duration for each individual frame from the Properties panel/Frame properties/Frame speed. The default speed is 1 which means that if the whole animation speed is 12 (in the Animations window, default animation), construct will play 12 frames in a second. If you set the frame duration (speed) to 2 on all frames, construct will play 6 frames in 1 second. You can mix and match the frame speeds to your liking, duplicate them and move them around and play back the animations looped and/or ping-pong looped...
Read the manual for more information.