To change wordpress logo/image from your wordpress log in page sign to your control panel. Go to File Manager ----> your wordpress instalation directory and then wp-admin ------> images directory/folder. Find and replace the image logo-login.png with your own image or logo. Save change and now go to your log in page to view your changes works or not. if not try to find out what you have done wrong. Tip: It whould be a good change not to remove original wordpress logo from images just rename original wordpress logo .e.g. logo-loginwp.png and name your own image logo-login.png, in case of any problem just rename your original wordpress logo.