OK, one step at a time.
I asked Google for help on this one and found:
https://www.samsung.com/uk/support/mobile-devices/how-do-i-use-the-gallery-app/This may look like the phone, but I suspect that it is the same for all samsung devices.
From your home screen with no apps open at all:
1 tap the gallery icon
2 Along the bottom there are the words: Pictures, Albums, Stories and then the three little vertical lines.
If you are already in the Pictures option, above the pictures but under the large word Pictures, on the right hand side there is the Magnifying glass (indicating search) and three little dots, arranged in a vertical line.
You see your pictures arranged by date taken: today, Yesterday etc and earlier dates by scrolling down.
3 Locate the picture that you wish to edit and tap it briefly.
4 Now you should see a large version of the picture, with a strip of thumbnails under it.
Under that are left to right:
a heart; a stubby pencil/crayon; three circles linked by lines (indicating share); the recycle bin; three dots in a vertical line.
See Roobarb's reply no 5 from yesterday for a picture of it.
5 Tap the short stubby pencil/crayon icon, 2nd from left
6 Now you can see the options to crop, change brightness etc.
7 At the top above the image on the right hand side are three more dots arranged in a vertical line
8 Tap those 3 dots and you should see a list:
Spot colour
Resize
About Photo Editor
9 Tap resize
10 You are now offered options of 20%, 40%, 60% 80% or original size.
11 Make your choice and tap Done
12 Above the picture, you will now see the words:
Revert
Save
and those three little dots in a vertical line
13 If you tap the three little dots again, choose Save copy. That way your original image is kept at the original size should you need it.
14 You now have a resized copy of your original image.
Hope that helps.
Nell