I'm not sure if the gallery downsamples images to 72 dpi automatically.. but just an FYI, if you send a jpg that's say, 4x6 inches at 300dpi, and someone views it in their web browser, the image size on the screen is determined by the combination of inches and dpi (every pixel will be displayed), thus a computer monitor set at 72 dpi will show the image at an effective 25 x 16 inches.. meaning a lot of scrolling would be necessary to view all of it, and you'd only see part of it at once
thus, it's always a good idea to downsize the image to 72 dpi at whatever actual viewing size you'd like it to appear as (but save your original high-res version on your computer, too!)