To upload, change or delete an image on your web page follow My Websites -> Main -> Pages -> Edit Web Pages and click the link to "edit" the page you want to change.
On the editing page scroll down to find the thumbnail of the image you wish to change. You do not need to delete the image currently displayed - just upload over it. If not replacing the image, tick the checkbox to delete the current file.
Click the "Browse" (or Choose File) button next to the image you want to to replace which will open a navigation window showing the content of your own pc or mac.
Navigate to find the image you wish to upload.
Click "open" to select the file.
If you have more images to upload on the same page repeat the steps above until you have all images selected.
Click "Save All" to upload your images and save changes to the page.