1. A method for overlaying an image in a video stream, comprising steps of: acquiring, from the video stream, an image element signature including at least one images element, determining whether the image element signature matches an image to be overlaid, overlaying the image when it is determined that the image element signature matches the image to be overlaid, wherein the step of acquiring from the video stream, the image element signature comprises: extracting an image frame from the video stream to be played, performing pattern recognition on a picture of the extracted image frame, and generating the image element signature based on a result of the pattern recognition, wherein the step of performing pattern recognition on a picture of the extracted image frame comprises: decomposing the extracted image frame into a plurality of sub-pictures at different scales detecting an object label and a probability value of the image element included in each of the sub-pictures at different scales, and recording an approximate position of the detected image element, and the step of generating an image element signature based on a result of the pattern recognition comprises: synthesizing the detected image elements in the respective sub-pictures at different scales on the scale of the original image frame, to generate the image element signature corresponding to the extracted image frame.