2/. In Photoshop
Check the images are all the exact same size.
Open the Timeline and ensure it has been set at - Create Video Timeline and not Create Frame Animation. The initial opening of the Timeline usually looks like this:
The eventual setting to Create Video Timeline is this, with a video icon left and a + sign right on the same layer.
Using the + select the images you want to include in your stop motion (there's no real limit to how many frames/images).
Using the play command, play your stop animation, to check everything is working properly.
Select the rounded right facing arrow, then this screen will appear:
Enter a file name and file destination. You may wish to change the frame rate. In this case a slower 25fps has been used. If you used a lower fps rate your movie plays faster. But 25fps is really good.
Go to the folder where your slow motion animation is and play your mp4 slow motion animation movie. This movie only plays on a local machine.