I cant change my profile pic using Gravatar on mobile..

I got the same problem

Sorry to hear that, I hope you'll find the problem, or that someone jump in this discussion to help you.

@zeropol 🤔still not working...something is wrong...

I see your pic as a black square indeed.
Gravatar should auto resize really big images ( >10Mo ).
(You should be able to crop the image and have a preview on the Gravatar website, maybe an extension of your browser blocks the javascript, or maybe the crop functionnality is not available on mobile.) forget this sorry, I see you already cropped the image.
example

edit: the changes you make on Gravatar can take a few minutes to be reflected here.

@zeropol 🤔I could try that.One more,even though I have cropped the picture,they would comeout pitch black..as my pp is literally black...I dont understand..does it have something to do with the pic resolution?or something like that?

If it is because the Gravatar website not displaying correctly on your device, you may try the web browser setting "show computer version of the page" ( I don't know how it is phrased in English, and the sentence may slightly change between different browsers. )
If it is for another reason, what is the reason ?