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I'm afraid, there is no option on HP Sprocket app to re-size an image, HP Sprocket only support the following paper and file size:
7.6 x 5 cm (3 x 2 in) HP ZINK® Sticky-Backed Photo Paper
BMP, GIF, JPEG, PNG, and TIFF (iOS only) file types
If you are using the file type and size listed above then HP Sprocket app should automatically adjust the picture to fill the paper size. However you can crop the photo on HP Sprocket App. Here is how to do it:
1.) On the HP sprocket app, from the home screen, swipe left or right to select a photo saved on your mobile device, cloud service, or social media account.
2.) Tap Edit or the Edit icon to edit your photo.
3.) Select Crop/Rotate
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