The evolution of a skin condition over time can be useful in its assessment. In an illustrative arrangement, a user captures skin images at different times, using a smartphone. The images are co-registered, color-corrected, and presented to the user (or a clinician) for review, e.g., in a temporal sequence, or as one image presented as a ghosted overlay atop another. Image registration can employ nevi, hair follicles, wrinkles, pores, and pigmented regions as keypoints. With some imaging spectra, keypoints from below the outermost layer of skin can be used. Hair may be removed for image registration, and restored for image review. Transformations in addition to rotation and affine transforms can be employed. Diagnostic correlations with reference image sequences can be made, employing machine learning in some instances. A great variety of other features and arrangements are also detailed.