Keep in mind that an unaltered photo from a phone or camera can be on the order of 5 - 6 Megs.
A pic that is about 500 kB is plenty big.
The easiest way to minimize a photo's size is by emailing it to yourself.
When you email it your mail client should ask you what size you want the attachment to be. Pick something less than full size.
Another possibility is the file type might not be supported. When I do "grabs" on my iMac they are .tiff files. I have to convert them to .jpg in order to upload them here because this site won't accept a .tiff file.