Here is a simple trick that will let you set a full image as display picture without cropping. Sometimes cropping of images can be annoying as some important part of image might need to go.
1. Download Es file Explorer (check Playstore)
2. Use Es file Explorer to open the folder containing the image you want to set as dp
3. Tap and hold on the image till its highlighted
4. Click on rename and change the extension of the image to "gif"
5. Now goto bbm and set the picture as dp... You won't the the prompt to crop/resize.
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