If you've uploaded a .png with a transparent background into a receipt, this could cause some display issues. You can easily fix this by saving your image as another file type.
Pretty much all photo editing programs will allow you to do this. Simply open your file in the program (such as Photoshop, Snagit, MS Paint, etc.) Then, click "Save As" and change the file type from .png to something else, such as .jpg or .bmp.
When you upload the image into your receipt, it should now display correctly.
If you're unsure of which program to use, you can try Pixlr. It allows you to edit images for free.