Is there a Dot-Probe Paradigm / visual-probe task available online?
There appears to be an Inquisit implementation of the dot probe task that will run both online and in a lab. Such scripts can presumably be customised to your purposes. 

If you want to use this for research purposes, you would need to have access to a licence to Inquisit. 

There is also a trial version of the software. You could download the trial of Inquisit and the script and run the script on your computer.