A person can get stock photos of flowers for free on Picjumbo, Pixabay, Gratisography, MorgueFile and FreeImages. Though free to use, some of the pictures may require an attribution to the original artist.
Picjumbo contains a database of free, high-quality photos. It contains a Search bar to find pictures of flowers, and the pictures do not require attributes. Most of the images on Pixabay also do not require an attribute, and it is easy to search for images of flowers on this site.
No attribute is required for pictures found on Gratisography, but the site does not have a search box. A person can find flower pictures on this site, but going to the Nature tab and browsing through the photos. MorgueFile does not allow a person to search for a specific object, but it also does not require an attribute. FreeImages also does not have a search box, and some of the images do require an attribute.