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.
A grey box will be used as default background for all users who haven't yet chosen their own profile background image.
If you're interested in expanding this default, vote for the feature request on Canny!