In this study, the existing optimal image thresholding techniques are reviewed for detection of painting cracks in general. Cracks on the painting can be detected by using the morphological top-hot transformation. However, since there are quite a number of false detections, a thresholding method should be applied to the obtained result. Therefore, optimal thresholding methods such as Otsu, Minimum error and Fuzzy thresholding can be used for overcoming this problem. Experimental results demonstrate the validity of the thresholding methods.