Since Linchpin User Profiles 2.21, it is possible to upload your own background to your user profile!
Enhance your profile with a background image
Navigate to your profile.
In the left upper corner of your user profile, you will find a Choose background button 
. Click on the Choose background button.
Now, click on the Choose image button. Select the file on your computer and upload it to Confluence.
Reposition your header image via drag & drop.
When you are pleased with the result, click on the Save button.
Enable profile background images
To enable this feature, navigate to Confluence administration → Linchpin User Profiles → Profile editor → Special fields.
Scroll down to the Background image section.
Select the Enable button to activate this feature.
Select the Disable button to deactivate this feature.
