There is a step-by-step explanation that people can follow if they would like to begin storing screenshots in their Dropbox. I linked to the instructions for Mac. You can also get a “how-to” for WinXP or Vista/Win7.
In short, you take a screenshot, then select to save that screenshot to your Dropbox. A window will pop up that gives you the option of saving that screenshot to your dropbox. (If you changed your mind, there is a “No, Thanks” button). Choose wisely:
From now on, whenever you take a screenshot, the application will automatically save it to a folder named Screenshots in your Dropbox folder. It will also copy a link to the screenshot that you can immediately paste anywhere, such as an email message or Facebook post. Anyone who clicks on the link will see your screenshot on a preview page on the Dropbox website.
In the same post, Dropbox also gives instructions about how to turn off the automatic saving of screenshots to your Dropbox folder. You can opt-out whenever you choose. In addition to using your Dropbox to store, and share, screenshots, there is another new feature for Mac users. You can now copy your entire iPhoto contents to your Dropbox. A new folder will be created for each of your iPhoto events.