Clockwork: error: cannot load 'recovery-clockwork': No error
Most common errors (and solutions to them) are:


different file name: make sure the file is really named recovery-clockwork.img, in your example, or adjust your command according to the file name
wrong path, e.g. your file resides in C:\Downloads, but you're executing fastboot flash recovery recovery-clockwork.img from C:\: Either first change to the directory the .img file is located in, move the .img to where you want to execute your command, or try specifying the file with its full path (e.g. C:\Downloads\recovery-clockwork.img)
sometimes adb/fastboot behaves strangely when executed from "somewhere else". In those cases it helps to first changing to the directory where the resp. executable is located, and run your command from there.