There really isn't much need to do that in my opinion. Broadband is quite common these days, and an image that size will load in a few seconds or less over most broadband connections.
Most WordPress themes will also only load the largest WordPress-resized version of the image which fits into the content area, rather than loading the full image and scaling it down.
Also, when you add the photo to your post, you have the option of inserting a thumbnail, medium, large, or full-size photo. The thumbnail, medium, or large will load the smaller WordPress-resized photos, and folks can click through to see the full version.
Another alternative is to use a CDN to serve the images. They're much faster as they are designed to only serve media files, and many CDN plugins also only serve the largest that can fit the content area rather than the full size. If you already have the Jetpack plugin installed, you can also just switch on the Photon module for a super-simple and fast image CDN.
Finally, if you still want to re-size them before upload, iPhoto has a handy Export feature under the file menu which will export all selected photos to the size of your choosing.