There are probably several ways to do it depending upon what software you have available. In Paint Shop Pro, I open the image file, then pick Image, Resize from the menus. Make sure Maintain Aspect Ratio is checked, then pick a suitable width (e.g., the picture I have in my sigblock is 142 pixels wide) and click on OK. Now check the Image Information to see what size the file is in RAM. If it's okay, Save As whateverfileMOD.jpg - if it's still too big, Edit Undo and try again somewhat smaller. Just don't simply Save as that will overwrite (lose) your original large image.

There may also be a way for you to change the pixels per inch from 300 (if that's what your original is) to the 72 that's all you need for monitor display across the internet. This will also shrink your file size, but may not shrink the picture. You probably have a Help menu command that can walk you through that if it's a possibility.