The average size of the images you use on your website does affect how fast it loads. If your images are large in size, it will take people with slower connections an eternity to get your website loaded properly on their computers. You can always optimize the images you use on your site for the web with tools such as Photoshop to reduce the overall size of your pages. Smush.It by Yahoo! is an online tool that can help with that too.
Smush It gives you multiple ways to add your files. You can upload them from your computer or provide the tool with their URLs (one URL per line). Once the optimization process is done, you can download the files to your computer to upload to your server. The tool shows you how much you have saved by using it.
Smush It is fast and does what it promises fairly well. If you are tired of your site being slow and want to reduce the size of your images, Smush It is worth checking out.