As you have probably noticed, each gallery has a cover picture, the same one that is displayed in the overview users get when searching galleries and on the gallery page.
You can change this picture directly from your back office, in the "My information" section, in the "Profile" tab. Click here to access it directly.
Then click on the + to upload the picture from your computer.
Please keep in mind that the profile picture should be exactly 530x330 pixels.
Do not forget to click Save at the bottom of the page to validated your changes.