I am trying very hard to change my profile pic. How can I do it? I think it is some reputation thing; is it so?
Reputation has nothing to do with you being able to change your profile pic. You should be able to change it, as often as you like, and to whichever image you want (with the obvious exceptions, cf. this meta post), regardless of your reputation.
For more info, see How do I change my profile picture, or avatar?. Quoting the relevant part,
How do I change my profile picture, or avatar?
For Stack Exchange:
- Navigate to your profile (click on your username at the top of the page)
- Go to the "Edit Profile & Settings" tab, and there will be "Change picture" link in the bottom of your existing avatar.
- Click on "Change picture" and a box will pop up giving you the option to choose between Gravatar, Identicon, your last uploaded picture, or uploading a new picture.
- Click Save changes just for this community to save the new avatar only for that specific site, or Save and copy changes to all Stack Exchange communities to save the new avatar to all sites you have account on. (Note: this will also override your whole information on all sites, e.g. About Me, email etc.).
- Images get updated quite quickly - you may need to clear your cache.