Obtain a sketchpad, either through trade, purchase, or one of the rare forageables, which are found in swamps if I recall correctly. (I've never found one, but I've not been that dedicated to foraging since they were introduced.) Upload your image, create the sketch, and then use the created sketchpad with the stand when you build it. Note (and this also answers the last question): if you do not include it in the building process, you cannot change it later.
I'm not sure if this will be helpful for you. The link includes the mesh used for the barter stand:
viewtopic.php?f=40&t=66553&p=841830&hilit=sketch#p841830 It's the best that I recall. If anyone has a better link, please post it.
Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.