Well, that may be changing a little bit now. Today I was working in Google Documents (updating a Chrome help guide), when I noticed a new option for inserting something called "Stock photos". Basically if you go to insert an image as usual, you now have five options:
I chose the "Stock photos" options (click on the image to the right for a bigger view), and then tried out some common search terms. I had pretty good luck getting images for most things I entered, however the results seem to be more like real pictures and less like cartoon clipart.
I then checked the rest of the Docs suite and found that the "Stock Photos" options is available for Google Presentations (the new version) and Google Spreadsheets as well.
This looks like a great step forward for Google Docs, especially in the education world, where teachers and students really appreciate the convenience of clip art or other stock images. Hopefully Google will be able to continue to add images to the stock photo archive over time.