On the road? At an Internet Café? Using someone else's computer? Your office can't afford Photoshop? Here is a overview of a few of the different image editing and creating tools available to make quick edits in your web browser. Note - 1) you'll probably have to mix and match tools to complete your task; 2) not all online tools are compatible with all platforms or browsers. Plan on tinkering a bit to get what you want.
Clipping is a handy way to collect important slides you want to go back to later.