Ranked in these QuestionsQuestion Ranking
Pro No ads
Although some images related to your search show up from sponsored web pages (notably shutterstock), they can hardly be labeled as adds. The section is well identified, related to the search, and does not make the page feel too distracting or unclean.
Pro Free API
Also, commercial use is permitted.
Pro Pixabay License
All imagery on Pixabay is under the Pixabay License (although material that appears within the images can be subject to copyrights).
Con A large number of unprofessional photos and especially vectors
The vector images can be quite unprofessional.
Con Users vote for pictures to get approved or not
Pictures that are not going against the "Quality rules" can sometimes get declined.
Since pixbay is a free user driven service, it can be difficult to find some pictures related to the subject one is searching on. While in other instances, a flood of results appear because it is a very easy subject to shoot, eg. Road (flood of options), Surprise (debatable options).