1. An image detecting method, comprising: capturing an original image; creating a moving-object image of the original image; creating an edge-straight-line image of the original image, wherein the edge-straight-line image comprises a plurality of edge-straight-lines; and detecting whether the original image has a mechanical moving-object image according to the length, the parallelism and a gap of the part of the edge-straight-lines corresponding to the moving-object image; wherein the step of detecting comprises detecting whether the moving-object image is the mechanical moving-object according to the sum of each product of a value of a relative length and a value of a relative gap of each group of the edge-straight-lines which are parallel to each other.