To prevent auto-zip, make sure the file has a file extension (it doesn't matter if it's hidden). Also, Enclose zips if the file includes a resource fork. This may be the case in your image files, where the resource fork contains a thumbnail or other info. This feature can be useful, but I can see why you wouldn't want it - it would be better to ignore the resource fork in this case.
As a workaround, you could use a utility to strip the resource fork before dragging to Enclose, such as
GrimRipperCM (contextual menu, freeware).
As to removing the image from the last file sent, it's true it never goes away. You could switch to the Sent or Files view using the toolbar (you can still drag new files to the lower window area). Or, close the main window, and drag the next file to Enclose's icon in the Dock (you can also reopen the main window using the File menu). If you just don't want the moving URL, in Preferences > General uncheck Animate URL. If none of these are satisfactory, contact me using the contact button and I'll see about getting a pref for this in a future version.