Nope, they can be .iso or .gcm...either way IMGBurn will burn them right to DVD. The mini DVD-R's hold 1.4GB, just like a real Gamecube disc. The only thing you'll need is a program called FSTFIX. Cube ISOs all have to be EXACTLY 1,459,978,240 bytes in size, or the Cube refuses to read the disc, so if you see some ISOs that are only 300 or so MB in size, they will work, but you have to use FSTFIX to put the dummy file back in so the Cube will boot them.