I do it like this on my iPhone: open Photos, pick the photo, tap Share → Add to Notes, then use Markup to cut out the part you want. Save it, and you can use it as a sticker in Messages by dragging it from Notes or using a sticker app.
To make a sticker on iPhone from photos, open the Photos app, choose the picture, and touch and hold the subject until it lifts. Tap Add Sticker or Add Sticker Effect from the pop-up menu. You can then use it in Messages or other apps that support stickers.
To make a sticker on your iPhone from photos, open the Photos app, select an image, touch and hold the subject until it lifts off, then tap Add Sticker or Add Sticker → with Effect. The sticker is saved to your emoji keyboard and can be used in Messages or other apps.
