Sketch Grid is a grid drawing app for iOS. It allows you to overlay a grid on an image and study the proportions.

You can export or print your image in the original resolution with the grid of your choice, and use that to paint or draw.

You can use a square or proportional grid, and make it finer or coarser to get the correct shape and line intricacies, or to get a general idea of the shapes and relationships in the picture.