Nothing beats free and Pixabay offers pictures and graphics with the CC0 license which is the least restrictive license possible. Simply put: take the image and forget about it.
Pixabay does not require attribution although I always require my students to credit their sources even when they are public domain. Pixabay does not require sign-in although they limit your downloads to medium size. The site comes in Spanish as well as number of other languages.
This is a fantastic site. My only minor quibble is that they also include Shutterstock photos which are not free. This might be confusing for some students.
If your students need pictures for their projects, Pixabay is a fantastic resource.
Description: Thousands of high quality pictures and graphics with no licensing restrictions.
License: CC0/no restrictions
Alternatives: Wikimedia Commons
Rating: highly useful