An e-shop designer theme for Ecwid consists of some CSS code that is copied into your Ecwid Control Panel, and some images that have been coloured to match your theme. The CSS refers to these images by their URL which will be made from the "Image URL Base" you specified, followed by the image name.
You need to store the images on-line so that they can be accessed by their URL. This usually means copying them to a directory on your web server using FTP, CPanel, the Wordpress Media Manager or some other way of up-loading files to your server.
So, for example, if your server is at http://www.example.com and you place the images in directory images/ecwid off the root for your site, you would use "http://www.example.com/images/ecwid" in the URL base" field.
You can always change the image URL base later by using the tool at http://www.e-shopdesigner.com/eshop_designer/css_changer/ . Paste your CSS into this page and set the new image URL base and it will update the CSS for you.
Of course, you can simply use a text or programmers editor to edit the CSS and do a global search/replace to correct all the URLs.
For more information, this Ecwid knowledgebase article may help: