Is it permissible to send a passport-sized photo to a company when applying for a job, knowing that I do not wear a niqab?

If you are in need of work, and the picture is limited to showing only the face, then there is no harm in sending it. This is because looking at the face of a non-mahram (definition) woman is permissible out of necessity, provided there is no fear of temptation. It may also be permissible to look at a woman's face when dealing with her in buying or selling or leasing, for the purpose of identifying her, even if it is disliked for someone who fears temptation or can do without the transaction. However, with necessity and absence of desire, there is no objection.