Once you have uploaded the texture you created in a paint program, (which will cost you 10 L) you can place it in world by going to a designated Sand Box area, creating a block/cube, adjusting the size of your new sign and then picking the texture that you just uploaded to apply to your new block/cube and there you have it !
Of course, you will not be able to keep it out there unless you have rights to leave prim's on the particular Sim.