Draw it up in paint or your editing program (GIMP is a good freebie) then upload it to an online storage like photobucket. (That's free too.) Once you do that, it'll give you a list of urls for various uses.... One is an img code. Copy and paste that into your thread, and... Tada, there ya go!
You didnt mention thaaat.
If its a smart phone, basically same plan, but using a free art app to draw it up. (DrawPad is an example on my Android phone...)