Need help with Monster Imports

I think my problem with this is from importing some pitcures that don't fit the standard sizes of the enemies I'm replacing- I'm just guessing at sizes, and I get a few that work and a few that are all messed in when I play. Sometimes the colors are all strange too.

I have a huge folder of new monsters that I spent a lot of time on, and I really want to be able to use all of them. Should I edit all of them to the closest compatible sizes or something? I'm not really sure what to do. Any tips and tricks I need to know when dealing with FF3SE?