Berlin-based artist Ellen Gronemeyer doesn’t often show her paintings. But that’s for good reason: Her pictures take time. She always works them over intensively, and her motifs emerge through this process. The small-format canvases in her recent show, “CDU/CSU” (named for the German center-right political faction), reveal that Gronemeyer, who is constantly developing and refining her artistic techniques, has brought her painting to a high level of compositional density. She presented just eight paintings and one drawing. Some are still, tight-lipped portraits; some retain a more cartoonlike or