With a page open with your image on it,
Window > Activity > find your image on the list, it'll give the KB size to the right, or drag it to your Desktop, select the image icon, Command + I / Right-click "Get Info", though it's better to do it from the browser since sometimes downloaded images change size as the OS adds stuff it needs to them.
You can find image size in Safari by right clicking on an image and opening it in a new window - the header/window name will be "image_name.jpg 150x150 pixels"
I get:
http://i1058.photobucket.com/albums/t401/blazywazy110/....jpg 26.4 KB, for example.