Based on the painting described, the most reasonable inference would be:
Since there are so many slaves working in the field, the cotton gin is not yet in use at this plantation.
This statement implies that the high number of slaves is necessary for manual labor in picking cotton, suggesting that if the cotton gin were in use, fewer slaves would be needed for that labor, as the gin significantly increases efficiency in cotton production.
The other statements do not logically follow from the visual information given in the description of the painting.