PHOTO EFFECTS + CROP WALLPAPER application is a solution of all your problems regarding croping images then set it to wallpaper plus photo effects.
-> This app will show you number of images in your phone in list
-> From that list you can go to your desire directory
-> Then you will have grid view of images
-> you can long press any image it will prompt a dialog from where you can select edit where you have following features
-> Auto Brightness
-> Photo effects
-> Tune Image
-> Or You can select any image just by clicking on it and it will open a image in new window where you can crop image according to your requirement.
-> Then set it to wallpaper and it will fit according to your phones screen (no worries of image pixels).