A possible workaround could be to get a photo gallery, then link from your gallery to your posts. In the gallery software, it automatically resizes your pictures for you when you upload them. There's even an option in the java applet upload to resize the images before uploading them.
Once your pics are in a gallery, you can go to pics, hold down the mac key and click on the pic then go to properties. Copy the link to the picture, and paste it into your post. The image will show up in your post sized correctly.
If you need more details, let me know. Here's some instructions on how to get set up with a gallery, and I will see about the Mac iPhoto program.