Patterns in Decker are "logical colors". Decker images can retain the appearance of 1-bit stippled textures across movements and fills because every pixel knows what pattern it belongs to. There's some information you may find useful in All About Color. Image formats like PNG, BMP, and GIF don't understand Decker's model of patterns, so round-tripping images through those formats loses information. Decker uses a text-based image encoding to store its images in the clipboard and in ".deck" files.
With scripting, you can write tools within Decker that import images in a 16-color or 256-gray palette and then re-map those to different colors or patterns. This would make it possible to draw images in an external tool using an appropriate palette to control which regions of your image become specific Decker patterns. Likewise, it's possible to write scripts to export images with an arbitrary 256-color palette, and you could use this to save patterns with solid colors another image editor can understand.
If there are specific things you find difficult to do within Decker, we might be able to offer additional advice. If you haven't seen it already, I highly recommend reading through Phinxel's Phield Notes for an overview of Decker's capabilities.