Jesterrace is right about the CD vs DVD distinction. Look at the back of the box the original came in, or at the disc itself. There will be either the "Compact Disc" logo or the "DVD" logo -- that will tell you what media you need to burn it on.
If it is a CD, in Alcohol 120%, you can choose PlayStation 2 as the datatype in the image making wizard. Then just burn that image to a blank CD-R.
If it is a DVD, you will have to choose media type. I've only done it a couple times for my PS2, and I've only used Taiyo Yuden DVD+R media with the booktype set to DVD-ROM. Both burns have worked perfectly. I've never tried DVD-R myself, but I've never heard anything bad about using it...
If I remember correctly, when you start the Alc120 image making wizard with the PS2 DVD in the drive, it just detects it as a standard DVD. Rip it to the HDD, then burn it. Easy!