Log into your Letsignit account. Then, click on your profile on the top right of your screen.

Click on "My profile" button:

Then, mouse over your profile picture and click on the "download" icon:

Now, you can import a new profile picture from your computer
(Drag it or import it from a file on your computer).

You can crop it if necessary.

Do not forget to save it. ;)

Once your picture saved, update your profile on the bottom left of your screen to finish the process.