Another company is using my images to advertise their work. Isn't this illegal?

When someone uses your original works (that you own) without your permission, that’s probably copyright infringement. 

There are degrees of “bad acting” here. Using your work to advertise is bad. But if they’re also selling your works to other people that would be very bad. 

First consider what you want. Do you just want them to take down the images? Or do you want compensation too? If the latter, determine the value of your work, which can be based off of your most recent sale or comparable works in your industry. You can also do some sleuthing — call this other company as a prospective client and ask about who took the photos on their website. They might provide you with a helpful admission of their own guilt.

Then, you can prepare a letter that demands the company take down the photos (and possibly compensate you for your loss of income or reputation). Be firm, yet professional. Make sure to include a deadline for them to respond by. 

If that doesn’t work, consider getting a lawyer involved or suing them. Depending on the size of your claim, it might be through the Civil Resolution Tribunal, Small Claims Court, or Supreme Court. 

Important caveat: if you sold the photos in question already (perhaps to a client of yours), then you might no longer own or control the copyright. Make sure you have a written contract with your clients that specifies what they are buying — it’s often best to sell licences of your work rather than the work itself so that you can sell multiple copies to multiple people. A consultation with a small business lawyer could be helpful here.