This paper introduces the concept of self-image-guided ultrasonic (SIG-US) wireless power transmission (WPT) for robust and efficient WPT to millimeter-sized biomedical implants distributed inside the body. In SIG-US WPT, a sharp pulse is transmitted periodically by the implant to create short ringing with relatively various delays across an array of external (wearable) ultrasonic transducers. These relative delays are used to drive the external array as in phased-array beamforming, generating a highly focused ultrasound intensity at the implant's location due to the reciprocity. Therefore, regardless of implant's misalignment, orientation, and medium (i.e., without any prior knowledge) optimal parameters for beamforming is found by the SIG-US technique without the need for a conventional imaging system, suffering from high power consumption, size, cost, and complexity. In our proof-of-concept simulation setup with a linear transducer array (11 transducers), the SIG-US technique improved delivered power to a 1 mm3 implant with 6 mm misalignment (powering distance of 30 mm) by 95.7 times compared with conventional beamforming. In addition, for up to 6 mm implant's misalignment, the received power with the SIG-US technique only varied by 1.2 times compared with 156.3 times variation in the received power in conventional beamforming.