Though there are sites where you can use images from (e.g. Flickr ELT PICS and others which you can find here), using clip art for worksheets or websites is still a great idea. Clip art can be used to create badges (e.g. if you use a LMS through which you can award badges to learners) and to brighten up any kind of worksheet (analogue or digital).
WP Clipart has over 50, 000 images organised in folders, making it simple to search for an image within a theme.
Clipartlord is another site which offers clip art for a wide range of themes.
The Association for Japan Exchange and Teaching includes in its resource page, a site where clip art can be shared.
School Clip Art, is still another source for clip art, dedicated to education.
Graphics Factory may be more suited to use when using for instance, a blog, or even a wiki, creating animated GIFs for education. (many make great interactive badges as well) If you use Windows, you may want to have a look here.
Three other resources include:
Edu Pic Graphical Resource,
Free Clip Art for Kids, Teachers
Humanline and ClipArt ETC.
You may also find further suggestions here:
Posters, Images and Metaphors
Blow Me Away with Images
Do you have any other Clip Art resources you would like to add?