When you use an image, you install Home Assistant OS as your base OS, there’s no Raspbian anymore. Home Assistant OS is headless with no Dekstop interface.
You’d need to install Home Assistant on top of your Rasbian, if you want to keep your desktop, through either Home Assistant Container, Home Assistant Core or Home Assistant Supervised. See here for a good overview of the different install methods. But in general it’s not a good idea to install HA as a background service on a desktop OS.