Valiant Entertainment began life as Valiant Comics back in 1989 and many of the characters who feature in their titles can trace their origins back to then.
The fictional shared universe has been rebooted since then, but characters such as Shadowman, X-O Manowar, Archer & Armstrong, Ninjak and Harbinger all continue to feature.
Unlike most publishers, Valiant Entertainment has an open submissions policy which can be found here; http://valiantentertainment.com/about-us/
Writers and artists can submit portfolios of previously published work to this e-mail address; firstname.lastname@example.org
However, if you’re still relatively unknown, it’s safe to say that you’re unlikely to get a response.
You’re best building up a body of work elsewhere and submitting to a company like Valiant later.
Valiant also continues to have a strong presence at Comic Cons. If you’re interested in working with them, then it is probably worth introducing yourself in person. You’re more likely to have success submitting to a comic book company if they can put a face to the name.
Although, if you’re going to try and make an impression, make sure it is a good impression.
Check on the Valiant website to see where Valiant Entertainment will be appearing throughout the year. They will have a booth and they will be doing Artist Portfolio Reviews at London Super Comic Con this month; http://valiantentertainment.com/2017/07/05/valiant-joins-london-super-comic-con-2017-with-exclusives-panels-merchandise-and-more/