Below are a list of free* websites which offer hi resolution images for you and your students - for multimedia presentations, posters, Google Slideshows etc.
iClipart (AEA Online Resource)
Creative Commons Search
Unsplash
PhotosForClass
LibreStock
MorgueFile
Negative Space
Pics4Learning
Pexels
Pixabay
Stocksy
StockSnap
FreePik
FreeImages
Gratisography