Why do people use CMS? they use it to avoid having to handle HTML structure, JS and CSS on a page by page base and focus on the content. But if you do want to handle them that way because it save you time, then maybe wordpress is not the best solution for you.
WordPress has to know what is the content of the pages to be able to function properly and image are part of the content (therefor having them in CSS is a no no), and you have to tell wordpress that it has an image at someway, with the most obvious one is to upload it via the upload windows. If you just ftp the image as PS probably does wordpress will not know about it.
Why is it important for wordpress to know about images? Because then the author can select them from the media library without reuploading them, automatic tools can generate thumbnail to be displayed at various place on the site and the image can be mark as a “feature image” to facebook or other 3rd parties.
The workflow with images in wordpress is to photoshop them before uploading the initial content.