Does your original disk actually need to be imaged?
You might want to check the Pasti database from this thread to see if that game has already been imaged to Pasti format or not. Of course, if you just feel like imaging up some disks then just pasti away.

Pasti or MSA/ST?
If you're absolutely sure that your original disk is physically unprotected, then an MSA or ST image will do as well (well, at least in my opinion

Please quick-check your Pasti images before uploading
This does not mean you should play them to the end of the game, of course. But please fire up your image on Steem (or any other Pasti compatible emulator) to see if it's booting at all.
Please post some info about the version of the game you have imaged
Is it a retail (full-price) version, or is it part of a game compilation, is it a budget release? This will help the preservation guys to file the image properly.
Useful links:
http://pasti.fxatari.com/
an instruction on how to image your disks with Pasti can be found in the "IMAGING TOOL for ST" ZIP file.
Pasti images that currently don’t work
Pasti images that should, but don't work
Titles that can't be imaged