Changing defaults for certain items?

[Johny_Bobman]

Hi guys,

I'm very much new to Ren'Py and I'm trying to figure out something I can't quite find in the documentation.
My basic question is, "How do you make certain things always happen, except for when you make exceptions?"

Specifically:
1. How do you make images with the tag "tc" always be positioned at truecenter? (i.e. tc fire, tc sky, tc tree will always be positioned at truecenter) and
2. How do you make everything (images and text) by default dissolve in, unless you specify otherwise?

Thank you in advance for the help!