Is there an alternative sitting posture in prayer for someone who cannot sit in the positions of Tawarruk and Iftirash? Please explain how this posture is performed with pictures.

Tawarruk and Iftirash are two Sunnahs in prayer, and sitting in a manner different from them does not invalidate it. Tawarruk is prescribed for the last sitting, and Iftirash for sittings other than that. Iftirash is sitting on the left foot and keeping the right foot upright. Tawarruk is spreading the left foot, keeping the right foot upright, and bringing the left foot out from under the worshipper to his right, while placing the hip on the ground. The evidence for this is the hadith of Abu Humaid Al-Sa'idi.