Windows used to have something called OLE I think it was that would automatically update an image if it were modified. I am not sure that feature is viable any more.

And to the best of my knowledge Xara cannot automatically update an image.

Any images you export (.jpg .JPG same thing) or PNG are going to export at 96dpi and if you check off Retina display, also at 192dpi. This is a standard for Windows display.

You can select your image and optimize it to export as PNG which will be sharper. Or if you have the option to create a .PNG instead of JPG I would chose that option.

But as far as replacing one spread sheet with another, I think you will have to switch out the images and republish.