I thought it might be useful to provide some details on how 16-color backgrounds can be imported as of Decker v1.12, which includes fixes and tweaks for a few minor but annoying stumbling blocks. There are many tools available for dithering images, and many algorithms and settings to choose from. In these examples I'll be using the command-line image manipulation utility imagemagick, because it is free, available on all popular operating systems, and easily automated.

Decker has an internal 16-color palette based on the 16-color Macintosh palette, with convenient named constants for each color in the global "colors" dictionary:

We'll need a reference image describing this palette in order to remap a full-color image. The easiest way to obtain one is to ask in Decker's listener. Execute this and save the result as "lut.gif":

r:image[16,1]
each c k i in colors r[i,0]:c end
write[r]

Then, choose an image of something beautiful, like a chicken:

Next we'll use the imagemagick "convert" utility from our operating system's command line. If we just "remap" our input image, we get a posterized result:

$convert gally.jpg -resize 512x342 -remap lut.gif output1.gif

A Floyd-Steinberg dither is much smoother:

$convert gally.jpg -resize 512x342 -dither FloydSteinberg -remap lut.gif output2.gif

An ordered dither is much grainier, but might be desirable if you specifically want to evoke a 90s aesthetic. I found experimentally that this image needs some contrast tweaks to avoid a washed-out result:

$convert gally.jpg -brightness-contrast -10 -resize 512x342 -ordered-dither o4x4 -remap lut.gif output3.gif

And of course there are many, many more parameters available to tweak.

Now that we have a dithered image, we can import it into Decker and splat it onto the current card background with the listener:

card.image.paste[read["image"]]

If our converted image doesn't exactly match the size of a card, we can go a step further by filling the card with black and then centering our image, for a nice letterbox effect:

i:read["image"]
card.image.map[() 1]
card.image.paste[i .5*card.size-i.size]

Now let's say we have a whole directory of wonderful chicken pictures (and who doesn't, really?) that we want to turn into a series of cards. We could use the above steps to prepare each one individually, but we could also use Lilt, the command-line counterpart of Decker, to automate the whole process, including invoking imagemagick:

What's particularly nice about this approach is that it's repeatable: at any time, you can re-run the script to regenerate backgrounds and add cards as necessary, while leaving any widgets, scripts, or other deck customizations intact.

Hope that helps!

I'm probably going to make a little alternative IDE for Decker as an OpenXTalk stack, and by that I mean I already started.
On mac,win,linux platforms OXT can call out to shell commands for stuff like imageMagick and ffmpeg.
It can also use embedded web ImageMagick (WASM) port to do image manipulation inside a Browser View Widget.
If running on macOS, OXT additionally has a wrapper library for some NSImage stuff with CoreImage filtering.
If nothing else it could expedite or offer fine controls for the lossy conversion to lookup palette images. 

Having Decker running in a browser view allows for tinkering around with the Decker html payload and then reloading it to test any modifications.

Also due to OpenXTalk having its roots in a Hyper-clone called MetaCard that dates  back to the early 1990s on Unix boxes, it also contains (still) a bunches of 16bit icon assets, as well as a set of 1-bit HC icons that a community member converted (I also made some SVG Path string versions of a few of those),  could add to the whole retro-ness.

Curious if there will ever be a version of Decker, or more features in Decker that are "color friendly" instead of it being sort of a hack/workaround to have color. I know this isn't totally in the spirit of Decker and HyperCard, but I really feel like it would probably encourage more users to gravitate towards Decker. Listen, I am 100% on board with the 1-bit Macintosh aesthetic, and I understand that it keeps the size of bitmaps small... But I feel like it would be nice to have the 16 colors not be so hidden. Let those colors be accessible from the toolbar. Have some sort of palette file, for swapping palettes between cards. 

Or... perhaps, if I could be even more bold, some sort of 8-bit color "extension" that allows 256 color palettes, but does not come stock with Decker? Perhaps also some sort of "extension" that allows for QuickTime style videos. I'm just sort of thinking about how it would be nice to author experiences with Decker that are on-par with other experiences of the Macintosh era, like Myst and Director-based games. Of course, this would balloon the size of decks... but this is why perhaps it could be some sort of "extended" feature. I just feel like Decker and Lil are powerful enough that they deserve to be at least this much more flexible as tools. I don't feel like this is feature creep/bloat to ask for. I think as long as Decker doesn't extend itself beyond "System 7" level Macintosh features, it will still be in the Macintosh spirit. 

I understand if this is not the ultimate goal of Decker. I just wanted to throw it out there. I still love Decker as it is and will continue to love it. I just would love to make experiences that are a little more 1995 than just 1985.