It is a great resource for free icons and png image archive. I found everything I was looking for my projects. Really very useful.
Vecteezy - An online marketplace where users can license stock photos, vector graphics, and stock footage directly from artists. Basic features are free but include ads and limitations. Pro subscribers get advanced licensing and a wider selection of content.
Pixabay - Over 270,000 free photos, vectors and art illustrations
Freepik - More than a million free vectors, PSD, photos and free icons. Exclusive freebies and all graphic resources that you need for your projects
Flaticon - A database of free vector icons.
Shutterstock - Shutterstock is a provider of stock photos, illustrations, and vector art. The website allows individuals to purchase a subscription and download copyrighted art for creative projects. Read more about Shutterstock.
PNGTank - Gallery of thousands of free PNG images with transparent background