The standard WYSIWYG editor in Xcart does not include a function for uploading images. You can buy an extension to the WYSIWYG and pay to have it installed, but if it’s not a frequent thing for you to need, then this is the cheaper way to do it.
In Xcart 4.6.x, go to Admin > Content > Edit Templates
Choose a logical folder to upload your image to
Upload the images
Check it in the address bar, and then copy that url
Go back to your static page and paste the urls into WYSIWYG editor using the image function
Here is a short video to show you how