Here you will find a vast amount of Apple mockups (PSD, Ai and sketch) and resources for designing for this popular device. Its regulally updated and even has the Apple Watch GUI ready to go for when you get your first Apple watch project.
Unsplash has become very popular over the last year or so with designers, the photos are of great quality and 10 new photos are added each day. I find myself struggling to choose which one to use for my wallpaper each month week. The down side to this site is that there is no search, hopefully a tagging system will come very soon but I do enjoy scrolling through all the lovely imagery.
Very much like unsplash, a selection of high quality images (110 as of this post) all royalty free. This site does feature a search which is helpful when looking for something in a hurry.
I sometimes forget about the IM Free site when sourcing images, they have a vast array of images all nicely organised into categories. Sometimes I am a little dubis of using some images and the rights to them but on each image page it states whether or not they are free for commercial use.
Getrefe site has a nice selection of images from a varied catalogue, it can be a little hit and miss with what you see on here, some photos I have noticed are taken from unsplash but its nice to look through every now and again for inspiration.
I came across this site quite recently, it has a huge resource of photos and videos all free to use in your projects. It probably the quickest to load out of all the sites that have been mentioned above.