I believe you can, but unless you have data on # of downloads they may not carry much weight. They generally have low threshold for acceptance. For photographers acceptance on free sites like Pixabay or Unsplash with # of downloads or on a site like Behance with #of hearts has more weight.
@Anastasia Y wrote:
I am an author of some pictures that are selling at stocks (like Shutterstock).
Can I use them in my portfolio as examples of my work?
Since you're selling those images (and presumably the rights), it could create problems depending on the usage licenses attached to anything sold on another site.
I suggest you create works specifically for your own portfolio so you don't have to worry about possible copyright issues with the stuff you're selling elsewhere.