There are many image photo sites available online. Some better then others. Most sites offer the ability to download an image. Sometimes you will also find the ability to choose different files sizes for the same image. If you are looking to produce a print its best to download the largest file size. There are many free sites online that you can research. Some are better then others. Most of these sites share the same images, so its more about the navigability of the site as well as the speed
Some free image sites require attribution.
Attribution is giving credit to the person or entity that created the photo . This is typically done by including a citation or credit line that identifies the photographer. The source of the photo and other pertinent info should be noted.
The purpose of attribution is to acknowledge and respect the intellectual property rights. Confirming who the photographer is and to provide transparency about the source of the photo. It also helps to promote the work of the photographer. This alone can be a way to build professional relationships within the photography community.