Well as it turns out, there might be a way to bypass that color conversion!
(For this method, I used Gimp, a free image editing program. You can probably get the same result with other image editing software, but this is what I use.)
Step 1: Resize your desired image to 512x512, 256x256, or 128x128. The maximum image size for uploading is 512x512, so if you want maximum quality for use on a knarr, you should resize to 512x512. Because of the way the game displays custom images, they might not come out right when rendered ingame. If you plan on using your image on a smaller object, like a shield or a painting, then you might want to resize to 256x256 or 128x128 instead so that it renders properly. The less detail you need, the smaller your image should be.
Step 2: Get a palette of all of ingame haven colors. Html color codes are: Red - #f01432, Orange - #dc5a1e, Yellow - #e6dc32, Green - #32b419, Turquoise - #5abebe, Blue - #2828e6, Purple - #be2882, White - #dcdcdc, and Black - #1e1e1e
Step 3: Convert the image from RGB mode to Indexed mode, using the palette you created and dithering enabled.
Step 4: Export the file so you can get an actually usable image.
Now you've got an image that won't be affected by the color conversion when uploaded!
Now enjoy your awesome barter stand or sweet kna- WAIT A MINUTE
Guess you gotta fork over some cash to Jorb and Loftar if you really want that nice knarr. (Store sketchbook pages can be used to craft a sketch for free, without the material cost or psyche requirement)
http://www.havenandhearth.com/portal/store