Robotic steering of magnetic guidewires has shown great potential in accelerating endovascular interventions, enhancing the success rate of time-sensitive surgeries such as stroke treatment. Incomplete state feedback of the guidewire from 2D perspective images and unknown interactions with the surrounding vessel wall raise challenges in modeling and steering control. These two factors, however, are commonly overlooked by existing works. In this paper, 2D perspective images of the guidewire, which comply with prevalent medical imaging modalities, are used as the only feedback. A model-based external force observer is proposed that allows the guidewire to perceive the unknown interactions, and a compliance controller is subsequently designed to handle the external force while steering the guidewire. Experiments conducted in a human-sized phantom demonstrate how the compliance controller preserves stability and safety by adapting to the estimated external force.