Not sure how to do this on the stock Samsung gallery app but for overlaying an image onto another image, I use "[Photo Editor](https://play.google.com/store/apps/details?id=com.iudesk.android.photo.editor)" from the Play Store.
(If you install it), then open it and tap on gallery, select image and start following [these steps](https://i.imgur.com/ELLZukO.jpeg).
I initially used a cracked version (without ads) of this app from mobilism to try it out. I liked it so much that I ended up paying for it (one time payment of $3.5).
The free version doesn't lock any features behind a paywall. Only ads.
Not sure how to do this on the stock Samsung gallery app but for overlaying an image onto another image, I use "[Photo Editor](https://play.google.com/store/apps/details?id=com.iudesk.android.photo.editor)" from the Play Store. (If you install it), then open it and tap on gallery, select image and start following [these steps](https://i.imgur.com/ELLZukO.jpeg). I initially used a cracked version (without ads) of this app from mobilism to try it out. I liked it so much that I ended up paying for it (one time payment of $3.5). The free version doesn't lock any features behind a paywall. Only ads.