Not sure what blue box you are talking about. Is it the background of a picture? If the image you are importing for your object has a lot of empty/white space around it, then you need to trim it before import, or make sure transparency is enabled if they are .png files...
including a screenshot or capx may help us as well.
If it is the collision box, you can edit that. if it is when selecting the image, you need to trim/crop out the extra space.