I want to use an image from this thread to my Thesis.
How can cite it.
The StackExchange network requires people who post questions and answers to grant them a licence: this is necessary because otherwise they could potentially be infringing copyright by serving up the questions and answers to other users. The practical implementation of this involves a licence called "CC BY-SA 3.0 with attribution required", and that licence effectively allows StackExchange to sublicense the content.
However, it's not an exclusive licence. If you want to use a single image for your thesis, it's probably worth posting a comment on the relevant answer asking whether the owner is willing to negotiate a separate licence for that purpose. I expect that most users would be honoured by such a request and happy to agree.
Two things to be aware of:
I'm not sure this is on topic for this site, but here's a link that may be useful: Citing websites and databases using harvard referencing.
The second one down 'To cite a page or document within a website' seems to be what you're after.