Decker's default window size is 512x342 pixels. If you draw pixel art in that size (or smaller) as black-on-white or black-on-transparent in some other tool it's fairly easy to import into Decker by dragging and dropping the file (bmp, png, gif, or jpeg) onto the Decker window or with the File -> Import Image... menu item. Decker automatically scales imported images down to a bit smaller than the full card so that resize handles are easily accessible, but if your image is already the right dimensions you can correct this while the selection is still active with Edit -> Resize to Original.
If you drag and drop or import a color or grayscale image, Decker will make a best-effort attempt at applying Atkinson Dithering to turn it into a 1-bit image. While the selection is still active you can resize the image and use j/k on the keyboard to adjust contrast, which may help make images more legible.
The process is somewhat more involved, but it is also possible to import 16-color images; quite a bit of discussion about doing so is available in this thread.
Decker can't display animated GIFs on a card out of the box, but there are some Contraptions that can add this capability, as well as the scripting APIs for decoding/encoding GIF framesto build variations on these examples.
Finally, it might be useful to know that with native-decker it's possible to activate Tracing Mode and use the built-in drawing tools to recreate anything you like from reference images. This can be a handy way to turn photos into something that blends in nicely with your own doodles.
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