I love working with printable images and graphics in my art journals. Here are some great sites where you can download images which can be used in your artwork.
The Graphic Fairy has over 5000 images to choose from.
Check out Wiki Media Commons: I love Wikimedia Commons and a lot of the graphics I print out to use in my journals comes from there! You can not only use the images, but enter photographic challenges, vote on images and more. Its a wonderful resource, 1000's of different images of all types.
If you like Clip Art then OpenClipArt.org is another great site for clip art type images. Everything is in the public domain and you can get items in vector format or png format.
Dover Publications Free Samples: Dover is well known for their clip art and royalty free images. You can subscribe to their mailing list for free samples. Ranging from coluring pages, clip art, drawings and more. It is all of a high standard as far as graphics go and perfect for using in your art journal pages.
Old Book Illustrations: All sorts of cute images from a wide variety of books. You could spend hours checking them all out. Reusable Art: This is an awesme site - wonderful description of the images. Easy to find and search for what your after.
PublicDomainPictures.net: While mostly photographs, there are a lot of illustrations as well, and all of the images here are in the public domain, which means you can use them however you like without worrying about copyright infringement.