Most photo management programs have a "compress" feature that will allow you to get below the 300kb requirement to post on MM. "Compressing" your photo will allow you to post the picture without having to reduce the photo size. Your resolution may not be good enough for reprinting the photo on a magazine cover, but it is perfectly adequate for webpage presentations.
For example, for photo files downloaded into Windows "My Pictures", there is a command for "Edit Pictures". This takes you to a command to "Resize", which will then provide an option to "Compress" to various resolutions. The "Document" option will generally compress a photo to between 175kb and 250 kp, all adequate to upload to MM.
Good luck.