Since you trained your model on mini-batches, your input is a tensor of shape [batch_size, image_width, image_height, number_of_channels].
When predicting, you have to respect this shape even if you have only one image. Your input should be of shape: [1, image_width, image_height, number_of_channels].
You can do this in numpy easily. Let’s say you have a single 5x5x3 image:
>>> x = np.random.randint(0,10,(5,5,3))
>>> x.shape
>>> (5, 5, 3)
>>> x = np.expand_dims(x, axis=0)
>>> x.shape
>>> (1, 5, 5, 3)
Now x is a rank 4 tensor!