Hi John, there's an explanation in the "Frequently asked questions - Rarely read answers" on how to attach photos and files to your posts.
For decreasing the file size, your best bet is to use a graphics or photo-editing program. Either resize the image to make it smaller, or save it as a file type (jpg for example) that requires less filespace. You can usually fiddle with the options when saving jpgs, to find a good balance between file size and image quality.