Both Decker and Lilt have a read[] function which can be used to read image files. The signatures are slightly different, since Decker prompts the user to open a file interactively while Lilt expects an explicit file path. When you load an image file, you will get an image interface, which, among other things, has a ".encoded" attribute which will give you the same sort of encoded image-string you get when you copy an image manually using Decker's drawing tools.
Let's say we're working in the Decker listener and we'd simply like to open an image, dither it, and then print the encoded string:
read["image" "gray"].transform["dither"].encoded
Or the same thing from Lilt:
read["path/to/image.gif" "gray"].transform["dither"].encoded
Web-decker supports a wide range of image formats- anything your browser can decode. Native decker supports GIF, BMP, PNG, and JPEG. Lilt supports only GIF.