Most likely that is the cover, and what you saw before was a temporary cover displayed until it fully downloaded.
However, it is possible to change book covers if your book is synced with iTunes.
Go into iTunes on the computer, find the book, right click on it and choose Get Info. Go to the Artwork tab. Whatever you put here will become the cover. Sometimes the ePub's cover is not stored here, but in my experience this will override other default covers. If there is already a cover, then you will have to remove it first. The ePub file will only hold one piece of cover art.
Look for your book on the internet, copy some cover art and paste it into the Artwork tab.
Now sync your device. The new cover should show up.
Note: I've never tried this with a DRM book, so YMMV.