Here are links to sites where you can find royalty free images, or content that is covered by the ‘creative commons‘ agreement, which indicates how content may be re-used.
pics4learning.com – this is a collection that has been specifically created by and for teachers and students for use in projects
flickrcc – images from flickr that are covered by Creative Commons licenses .
compfight – similar to above
flickrcc – similar to above
Clipart Etc – Florida education project – Victorian woodcut like clip art – great for collage
open clipart.org – is what is says
cogdogroo.wikispaces.com/StoryMedia – link to a whole wiki of further resources
commons.wikimedia.org – quick and easy way of finding images that are covered by CC licences
free digital photos – lots of photos, generally free if you only use small size images
the full montage – resources of collage artists – beautiful cutouts, but you have to pay a bit for them
textures – free textures for use in photomontage and 3D rendering
cgtextures – as above, but you have to create an account