High-end photo labs and desktop publishing service bureaus offer scanning services. If your original is large, it can be scanned in sections and later be 'stitched' back together using image- editing software. Now you need a web page. If you don't know how to build your own, have someone create one for you, or put your work in an online gallery. If you want to have complete control over the design, you can take classes in website design.
Images are slow to download.
Your visitor may get tired of waiting and move on. Thumbnails can be linked to full-size images, and will give your visitors the option of choosing which piece they want to look at first. If an online gallery accepts your work, they will design the web pages. They may accept photos, or ask for the images to be supplied digitally. Some art organizations, like Artist Resource or Art.