This app allows you to resize your screenshots for publication in the AppStore. And you can use custom resize.
You can choose preset:
-iPhones and iPads
-iPhones only
-iPads only
-Mac OS
-Custom size(set Width and Height)
You can save resized screenshots in one folder. Each time, a + 1 index will be added to the new screenshots name.
How to use:
1) Upload your screenshot .jpg or .png
2) Select a preset
3) Choose a folder in which resized screenshots will be saved
4) Enjoy