PNG-File shoult no have an alpha channel

Good work. Thank you very much. Helpful for many a photo amateur.
Unfortunately I have no idea in Python programming. But tensorflow gives the error message: 

"ValueError: Cannot feed value of shape (1, 252, 261, 4) for Tensor 'Placeholder:0', which has shape '(?, ?, ?, 3)'" 

if the PNG file contains an alpha channel.