All food and garlands as well as any other “offering items” should be offered to Lord Krishna and/or Lord Caitanya BEFORE being offered to Mukunda Goswami. These items should preferably be offered to temple or home Deities. Photos of paintings of Deities are also acceptable. Since temple and home Deities are usually made from stone or wood (including marble, other metal, such as obsidian, bell metal and ashta dhatu [a selective mixture of eight metals: copper, brass, tin, lead, iron, gold, silver and steel, used by the sthapatis in the making of Mayapura’s Panca Tattva Deities] and other stonework, shaligram- and govardhana-silas, neem or other wood) and since Deity paintings and photos are relatively commonplace, these are more acceptable than Deities made of soil, sand and jewels. Only in emergency situations, such as the guru or sannyasi being offered the prasadam having to leave the premises immediately and suddenly, should offerings be made in the mind.
According to Srimad Bhagavatam 11.27.12, Dieties can manifest in the following eight substances: stone, wood, metal, earth, paint, sand, the mind or jewels.