Hi there!
I don't see any 404's on my system.
I don't see any references to images in postman-smtp/script/jquery-steps/jquery.steps.min.js
I don't see any references to images in postman-smtp/style/jquery-steps/jquery.steps.css
and finally there is no mention of images at http://www.jquery-steps.com nor are there any images in the source code at http://www.jquery-steps.com/Download/SourceCode
I have no idea what images you add manually, but they are not required by Postman, nor the stock jQuery Steps.
I'm going to guess and say you have loaded an extension on your system that is modifying JQuery Steps to have images. This extension should not be running on Postman's page. That's not correct.