What does this do?!
It converts .png, .jpg, or .gif (non-animated) images into .json files for import into Starbound using Starcheat.

Basically this converts an image into signs that can be used even in multiplayer.

What's required?
Latest version of Java Runtime Environment (http://java.com/en/download/)
Starcheat (http://starcheat.org/)
MSPaint or Photoshop or another image editing program of your choice

How do I use this?

• Create an image whose width is a multiple of 32 and height is a multiple of 8.
  The multiple doesn't need to be the same - you can have a 32x80 or a 64x8 image for example. There is no limit as of now to how big your image can be unless your height or width is bigger than say 2,147,483,646 pixels... which is ridiculous but don't try it please.
• Save that image as preferably a .png, although again .jpg and .gif (non-animated) work too. Transparency works!
• Open up SBSignConverter.jar, click "Open Image" and select the image you created.
• Select the self-explanatory options. Removing the frame and background and using a transparent .png or .gif will allow you to make something that doesn't look like a rectangle!
  

• Click "Create Sign(s)".
  This will make a new folder named after the image you selected in the same directory as that image, then place a bunch of .json files there. The .json files are named x#y#.json, which tells you where to place the image. x0y0 is the top left corner, x1y0 is to the right of x0y0, x0y1 is to the bottom of x0y0, and x1y1 is to the bottom right of x0y0, etc.
• Import the sign .json files into Starcheat.
  Open up Starcheat and select your character. Right click an empty inventory slot and click "Import". Select the first .json and hit "Open" then "OK". Repeat for all of your signs. There's a limitation where you cannot modify the description nor name of a custom sign so once in game, the only way to know what sign is what is by where you placed it in your inventory, besides the tiny inventory icon of the sign itself. Therefore I suggest arranging the signs in a way that makes sense to you for easy placement. 
• Save your character in Starcheat, open it up in Starbound, and place your new signs!
  

Thank you to Credey and Poppy for helping me test this!

Latest changes:
Updated the UI, added the ability to give the sign inherent lighting properties and to remove the frame and/or background. Again, if you remove both the frame and the background, any transparent pixels in your image will actually be transparent in-game too, allowing you to make signs that don't look like a rectangle. So perhaps you could make furniture (that's unusable), lights, or basically anything.