I just wanted to know if I am allowed to use wallpapers from free services like unsplash that does not need license for commercial and non-commercial purposes in my portfolio or not
Your portfolio must demonstrate YOUR capabilities… if you aren’t using your own assets, your rejection is 100% guaranteed.
Once approved, you can use these free wallpapers like a lot of sellers do, but not for your initial application. Good luck!
Assuming you are already an approved theme designer . I suggest you include a link to the specific licensing agreement. My feeling is that the team will reject without that specific notation and assume that all stock images are licensed unless proven otherwise.
Samsung Developer Relations