Answers (2)
1 - Pack it in a rar/zip archive and use the high compression option which will decrease the size of the image, then e-mail it. The reciever can download it and extract it.
(zip archives are supported by most devices)
2- or use a software such as format factory to change the quality of the image (the image quality will drop which will have a bad effect on it espicially if it has details that require a high quality to see)
Almost any viewer will load a file in one format and store it in a different format. If you reduce the size and change the format, the size of the file is smaller. It's hard to predict the results, so you have to try it to see what happens. Back in the BBS days with dialup modems, sysops would routinely turn jpg advertising images into gif images to reduce the file size to 10K or so.