To add a data payload via OP_RETURN, you have to add an additional output to a transaction. This requires 11 bytes of transaction data overhead. The data is not subject to segwit’s witness discount.

Vojtěch Strnad shows that inscriptions have at least 118.75 vB overhead. However, the data payload of inscriptions is subject to segwit’s witness discount.

According to Strnad‘s calculation, data payloads of 143 bytes or larger are cheaper with inscriptions. It would therefore only make sense for small payloads to move to OP_RETURN, and it seems economically unattractive to prefer OP_RETURN outputs over inscriptions to embed images in the blockchain.