FreePik - FreePik aggregates free stock vector artwork, photos, icons, and Photoshop design templates from around the web. Their generous licensing terms usually only require writing "Design by FreePik.com" in a discreet location on your work, such as at the bottom of an app description.
ShutterStock - Shutterstock sells stock photos, vector and illustration artwork. It is ideal if you have an established business with a need for a high volume of high quality graphics. If you are in this position, a good option is to purchase a 1-month professional subscription, which will allow you to download 750 images for a total of $249. Over the course of the month after you subscribe, plan out your next 20-40 apps and download all the images that you foresee needing to produce them. Then, when you have a few days remaining, cancel your subscription. You can re-subscribe at a later time when you have more projects planned that you need images for. If you are not sure what to do with your 750 downloads, look at your competitors' businesses and notice the images that they use in their work. You might benefit by finding similar images on Shutterstock, in case you want to use these images at a later date for future projects.
Pixabay - You can find over 500,000 free stock photos, videos and illustrations that you can utilize for any commercial purpose, with no attribution required.
Unsplash - Unsplash offers hundreds of artistic photos, free for unlimited commercial use. It is one of the most incredible photography resources on the Internet.
Fotolia - You can purchase 5 images for $25 for use on your blog, website or app. Choose from 55 million photos, vectors and videos on Adobe's stock marketplace. Alternatively, you can also purchase 5 images for $39 on DepositPhotos, or buy 5 images for $50 on Shutterstock.
Flat Icon - Flat Icon provides over 100,000 free vector icons. You can download a pack that fits for your app aesthetic. If the icon pack does not have a few icons that you need, you can easily create a new one by sourcing vector elements from another icon on the website.
365 Web Resources - This site provides a potpourri of design resources for digital marketers. Most of them are free for commercial use.
Adobe Color CC - A beautiful color scheme is a necessity for any quality app. Never worry about coming up with a great color scheme again. Browse through hundreds of beautiful color pallets and choose the perfect one for your app. If you still don't see a color scheme that meets your needs, Color-Hex is another good source.