When you are emailing from your iPhone and attach a picture as enclosure made by your iPhone you have, before sending out the email, the option to shrink the pic to a smaller size. I always use "normal"which is around 130Kb so will not take that long to upload even with a slow internet connection.

 

Check your settings on your phone, I do not know about Apple stuff but you should be able to open your picture and check how many megabytes (mb) or kilobytes (kb) the image is. Most of my images are set at 3264 x 2448 pixels at 300dpi (dots per inch) and most range around 1.3mb to 1.5mb which I believe are under the allowable size to upload here on the Forum.

You can set your pictures at 72dpi but if you ever wish to print out a large image and frame it then it will appear fuzzy and lose clarity/quality in its appearance, but I know some phones/cameras with higher pixel ratings you can save mega large images, so try and load your image onto your PC then right click on it and select Properties, then go to Details and check where your settings are.