NO PORN OR HATE OR NUDITY IS ALLOWED ON SIGNS! THIS IS A BANNABLE OFFENSE!!
You can add images from the web to signs in game using the /sil command.
Simply type: /sil <URL of image>
To add formatted text, is a little more involved, but is not difficult.
The format for the command is:
/silt <text> <font size> <text color> <background color>
Replace <text> with whatever text you want on the sign. If there are spaces in the text, be sure to put it inside double quotes.
Replace <font size> with a number. You may need to play around with different numbers to find the size you like best.
Replace <text color> with a hex color code to change the color of the text.
Replace <background color> with a hex color code to change the background color of the sign.
Hex color codes look like this: #00FF00 and can be found using this site: Hex Color Codes