Is marketing via Instagram considered forbidden if it involves the use of hashtags that may lead to images unlawful to view, thereby making me a means of spreading the unlawful, and does this prohibit what I earn from it?

There is no impediment to publishing products via hashtags, as long as they are not designated for prohibited items. In this case, aiding in the viewing of prohibited items is indirect and unintended. Publishing products in forbidden hashtags does not affect the lawfulness of the money earned from their sale.