Download the texture, customize it on your computer - Microsoft Paint can do the job easily! After that,
upload the image into Second Life, get the texture's UUID and set the texture using the script in the
uniform or simply by using the build tools.
In order to apply the patch after upload, you need to go to the uniform's menu (see manual for details),
PATCHES and type the UUID of the freshly uploaded texture. See the picture below for reference: