1. Open your gallery and tap on any image you would like to change
2. Once you do that you will then want to tap the “edit” option
3. You will then see an option that says adjustments click on this
4. Once you do that you will be provided with a list of different options such as rotate, flip, flip vertical, and ratio
5. For how to change picture size on galaxy s8 / galaxy s8 plus you will want to tap the ratio option, and choose whatever size your looking for. Whatever the size you need it will be in this section. Once you have chosen your size click apply and then save