If you are supporter (which seems to be in your case)
Click on your badge on the team overview page.

A new page comes up which says:
"Image can be maximum 140x140 pixels in dimensions (if the image is wider or higher then it will be automatically stretched). Also, the size of the uploaded image file cannot exceed 100 Kilobytes."

Under this you have an input field, a seach button and an upload button.

Click on search and you can search for your local stored picture.

When you've found your picture, click "open" button.

Now the location of the picture should be seen in the web page input field. Now click the "upload" button.

If you managed to match all requirements, your batch is stored.

Now open your team overview page and if it is NOT there, press F5.
The local store has to be reset.