The speed adjustment works only for video, not still images. That is because the speed adjustment speeds up or slows down the frames per second of a video, whereas still images do not have frames per second. To slow down or speed up the display of a still image you adjust its duration (the amount of time it displays in the timeline). So, for example, if you want to "slow down" a still image you would increase its duration. If you want to "speed up" a still image, you reduce its duration. You can increase or reduce duration by dragging the edge of the still image to expand it out or reduce it down. Alternatively, you can select the still image, or select a range of still images, and then click on the information icon (the circle with and "i" in it) in the tool bar at the upper right of your screen. A duration box will appear to the right. You can type in whatever duration you want. Then click on the return key on your keyboard. That will change the duration to the duration that you typed into the duration box.
-- Rich