Cardiac vector projection forms the basis of 12-lead electrocardiography (ECG). However, the spatial relationship between the leads is not obvious in 12-lead ECG. In addition, the lead strengths of the three augmented leads are insufficiently represented. In this paper, we propose a new image ECG, which represents the cardiac vector projection over a 360 degree spatial angle on an observation plane (frontal, horizontal, or sagittal plane) in 3D space. We also present a novel 'cardiac vector circle' and mathematically demonstrate that the diameter of the circle equals the magnitude of the cardiac vector. The intersections between the 'cardiac vector circle' and ECG leads determine the projected ECG lead strengths. We further demonstrate that the lead strengths of the three augmented leads of 12-lead ECG are only V3/2 that of the projected lead strengths; this finding shows that the lead strengths between the standard bipolar limb leads and the augmented limb leads are not on the same scale. Finally, we also illustrate that there exists an evolutional relationship between 12-ECG, vectorcardiography (VCG), and image ECG.