Select your main image that is your 'Background' Imagine. Load it onto photoshop
Select the image that you are going to be overlapping your background image with and copy it as a layer so you can paste it onto the central image.
Select a lower opacity on the layer image so you can match up the two images, so they match and all the body images are in place, you can use Command T to do this.
Select a low opacity and save one image as '1' twice, in formats 'JPEG' as well as 'Photoshop'.
Repeat this, increasing the opacity gradually and saving the image each step of the way in both formats.
Go on Image and select 'Scripts' then 'Load Files into stack'. Load the JPEG versions of all the images you gradually saved. This will load them in your Sidebar.
Press 'Create Frame Animation' in your tool bar to start creating your animation.
As you add each layer to the animation, hide the layer above and then add it to create a slide in your animation.
After you do all this, your animation will be created, and you can crop your image, change the pace of your animation etc..
Finally, to upload your GIF to the internet and keep it as a working gif. Press File - Export - Save for Web (Legacy)..
And then save it.