A method for encoding an image based on convolutional neural network is provided. The method includes steps of: a learning device a learning device including a first to an n-th convolutional layers, (a) obtaining at least one input image; (b) instructing each of at least one of the convolutional layers to (i) apply one or more transposed convolution operations to the input image or an input feature map received from its corresponding previous convolutional layer, to thereby generate one or more transposed feature maps which have different sizes respectively, and (ii) apply one or more convolution operations, with a different stride and a different kernel size, to their corresponding transposed feature maps, to thereby generate their corresponding one or more inception feature maps as a first group; and (c) concatenating or element-wise adding the inception feature maps included in the first group to thereby generate its corresponding output feature map.