Case 1: Format difference
It worked when i changed the images from jpg to png, using Preview. Previously It might not be working due to .png as name only not as format. This link helped.
Also to convert the multiple images to png in a folder in single step use terminal. Using cd goto the folder containing the images (JPGs or any type). run Code:
mkdir pngs; sips -s format png *.* --out pngs
It will convert your images to .png and will create a pngs folder contain the converted images.
Case 2: Two images with same name or one image added twice:
As Rajneesh071 & Himanshu suggested: There can be two or more images with same name or two targets were added for same image. Goto project target ->Build Phases -> Copy Bundle Resources and search for your image file name. and remove the repeated file.