NOTE: Change of the default wallpaper is currently not possible on our hosted accounts
In order to create a copy of the Blue Aero affiliate panel theme navigate in the merchant panel to Configuration > Design > Affiliate panel theme > click "Edit theme" on the Blue Aero theme. You will see the Create new theme dialog that allows you to change the name of the theme. Name it e.g.: Custom Blue Aero theme.
Once you click "Create" the Theme editor will show up. Exit the Theme editor and click the "Set as default" button on the freshly created theme.
In the confirmation dialog check in the "Apply also for existing affiliates" checkbox and click Yes.
Now, access the installation files of Post Affiliate Pro and upload your custom image file into
/accounts/default1/themes/affiliates/_common_templates/img/wallpapers/
Afterwards, navigate to
/accounts/default1/themes/affiliates/Your_Copy_of_BlueAero/
and open there the theme.php file. In that theme.php file change "PostAffiliate_Galaxy.jpg" to the name of your image file , so that instead of "defaultWallpaper=PostAffiliate_Galaxy.jpg" you have e.g. "defaultWallpaper=mycar.png"
From this time on, whenever the affiliates access the affiliate panel they would see yoru copy of Blue Aero theme with your custom wallpaper.
In order to prevent affiliates to change the theme of their panel you can remove the Panel Settings menu item.