Resize image within photo

[bsnmate]

Hello. New to forum. I would like to know how to resize an image within a photo file without reducing the whole photo. I have not been able to get an answer to this question with multiple searches. Most posts have to do with reducing/compressing the entire photo file, and this is not what I want to do. I want to shrink an image to make a more usable wallpaper. I am new to photo editing so am very ignorant of terms so please be gentle. Thanks