There are two aspects here, one the actual physical size of the image - it does not need to be any bigger than the size you want to see on screen - some cams produce sizes of 30" * 20".
Secondly if you saved it in JPEG format you can then reduce the image quality with the right software without affecting the web view.
How do I reduce the size of photos for online use?
+ Google may help. I'm not sure which free image progs will do this, but someone will know!