Never make a stock image bigger. It expands the pixels and reduces quality. Try and find either a bigger version of the picture and minimise it or try and make do.
Use the zoom tool (press Z or find it on the list, it's below the very bottom selectable tool, or bottom right if you have the panel doubled up) when rendering an image also. Makes things alot easier
With a plain white background, you can cheat and use the magic wand tool (if it works great, but most of the time it's useless) or use 'colour range' which is under the select tab at the top