Collage.com's crop tool can be found by clicking the photo that needs to be cropped and then clicking the crop button that appears below the selected photo.
This will open the crop feature. Use the white circles on the edges of the photo to drag in and out for cropping and then click "Done" when it is cropped as desired.
You can easily undo cropping here by reselecting the cropped photo and then adjusting the bars back to their original position.
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