Keep anything that supports a figure on your return: wage statements, 1099 forms, interest statements, receipts for deductions and the return itself (sample note).
We suggest one folder per tax year, paper or digital, and keeping it for at least seven years. Letters from the tax office go in the same folder, with a note of how they were answered.
Photos of receipts are fine, as long as the date, the amount and the seller can be read.