Image retargeting techniques that adjust images into different\ud sizes have attracted much attention recently. Objective\ud quality assessment (OQA) of image retargeting results\ud is often desired to automatically select the best results. Existing\ud OQA methods output an absolute score for each retargeted\ud image and use these scores to compare different results.\ud Observing that it is challenging even for human subjects\ud to give consistent scores for retargeting results of different\ud source images, in this paper we propose a learningbased\ud OQA method that predicts the ranking of a set of retargeted\ud images with the same source image. We show that\ud this more manageable task helps achieve more consistent\ud prediction to human preference and is sufficient for most\ud application scenarios. To compute the ranking, we propose\ud a simple yet efficient machine learning framework that uses\ud a General Regression Neural Network (GRNN) to model a\ud combination of seven elaborate OQA metrics. We then propose\ud a simple scheme to transform the relative scores output\ud from GRNN into a global ranking. We train our GRNN\ud model using human preference data collected in the elaborate\ud RetargetMe benchmark and evaluate our method based\ud on the subjective study in RetargetMe. Moreover, we introduce\ud a further subjective benchmark to evaluate the generalizability\ud of different OQA methods. Experimental results\ud demonstrate that our method outperforms eight representative\ud OQA methods in ranking prediction and has better\ud generalizability to different datasets.