Stock Photos, Clip Art and Icons
Pixabay, Free Images and Morgue File search their own databases of photographs. Every Stock Photo searches several other free sources.
The images on these sites are free, but they are also ad-supported. Images from professional stock images sites are usually integrated into search results. Students are likely to accidentally click on the paid results. These pictures are watermarked, but there still might be some frustration.
The Noun Project has user contributed symbols and icons. They would be great for infographics or logos. The only issue is that if you want to use them for free, you have to give credit to the designer. Otherwise, each one is a few dollars each.
The images on Open Clipart are also user contributed and are free for commercial use.
Pixabay also has vector graphics and illustrations in addition to its stock photos.
General Search Engines
The images you can find through general search engines have their own issues unless you can identify which ones are are cleared for commercial use.
Luckily, there is a usage rights filter in Google Images, Yahoo Images and Bing Images. Just perform the search as usual and then look for the filtering option right underneath the search box.
Here is an example from Google Images.
Bonus: Other Tools
Here are a few other tools that might also be useful for design projects.
Coolors generates random color palettes that you can fine tune and then export the color scheme. The pallet includes the hexadecimal codes you can use in your favorite image editing program. You can also browse other color palettes that other users have generated. Colourlovers is another user contributed site where you get inspiration from other peoples color palettes.
There are also few useful Google Chrome extensions that might prove useful for student designers.
Use What Font to identify the different fonts used on a website. With Palette for Chrome you can right click on a image on a website and get a color palette that matches the picture you selected. Do you just need to know the RGB or hexadecimal value in one specific image? Then install the Color Pick Eyedropper extension.