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