I use shrinkpic on my pc, not sure if it works on a Mac. This software is free and runs in the background, automatically resizing images when uploading to web or e mail. You should have photo editing software and it probably has an option to save images in a format and file size suitable for the web.The file size is limited for several reasons, upload and download time and storage space on the server.

I have Samsung, the photo size can be altered in 2 seconds connected the camera to computer and uploaded straight from the cameras memory card or storage. I use photo size for Email, no need to transfer the photo to the computer unless you want to.