You're still not making much sense to me.
To replace a cover image with one you made outside of calibre use the Browse button in Metadata Edit->Change Cover to go get your image, or just drag 'n' drop the image onto the existing cover in the Book Details panel. When you send your book to your android device your cover will be used in the EPUB it sends.
If you sideload your Android then either do a Save to Disk and send that file, or if you want to sideload from within the library (as I do) then you'll have to Embed, Polish or Modify the EPUB within the library first.
Added - I just noticed from your other thread that your talking MOBI in FBReader - it which case all bets are off, that's a road which I choose to avoid
BR