It looks like the problem you are experiencing is failure to correctly use the command line and filesystem. This isn't bad, just inexperience like you have stated.
For starters, it appears that you have the unecm command installed. If I understand correctly, the compressed CD image is located on your desktop (the TonyHawksProSkater2.bin.ecm file, henceforth known as the ECM file). You will want to change directory to your desktop.
It appears you attempted to change directory to your desktop in the first command, but didn't read or understand the output.
The second line is an error message, because you have forgotten to include the cd command in front of your path. The first command should look like this:
bash: /home/haroun/Desktop: Is a directory
Remember, as you type paths into the terminal, attempt to use the Tab-Autocompletion feature to ensure that you do not accidentally type the filename wrong.
If successful, your command prompt should change to look like this:
The ~ is shorthand for your home directory, /home/haroun. You can use it in commands and paths if you don't wish to bother typing the full path.
Finally, the next command you ran was mostly correct, but since the first failed this one failed as well:
You did not need to specify the full path to where you wanted to save your file, the /home/haroun/Desktop/TonyHawksProSkater2.bin.iso part. However it doesn't hurt, it just would default to using the current working directory if you left out the /home/haroun/Desktop/ part. Finally, do not append the .iso extension to the end of the file. Although you might be most familiar with ISO files, which is a type of disc image, there are MANY different types of disc images and you cannot simply change the extension to another format.
haroun@harouns-MacBook:~$ unecm TonyHawksProSkater2.bin.ecm /home/haroun/Desktop/TonyHawksProSkater2.bin.iso
UNECM - Decoder for Error Code Modeler format v1.0
Copyright (C) 2002 Neill Corlett
Decoding TonyHawksProSkater2.bin.ecm to /home/haroun/Desktop/TonyHawksProSkater2.bin.iso.
TonyHawksProSkater2.bin.ecm: No such file or directory
It appears that your disc is using the BIN type disc image, as evidenced by the .bin part of the filename. These are commonly bundled with another file with the same name but with a .CUE extension, which isn't always needed.
You could use the BIN to ISO converter, once you have managed to decompress the ECM image to get the disc image into an ISO format, but there's no point. Playstation emulators for the most part understand the difference between BIN and ISO and do not require you to convert it. Finally, playstation discs generally cannot be accurately represented by the ISO format and may break your game if you try.
Hopefully this wall of text will help!