Just wondering, if I use an image made out of thinned RI (like color flow) how long will it be ok to sit on top of buttercream? A few days? I don't want it to dissolve before they serve it so just wondering. It will be a crusting bc

I wouldn't risk to set it on top of buttercream. I would cheat and create a barrier between the two. Simplest would be to take four sugar cubes, set them in the buttercream, leave a small bit over the buttercream and set the image on the sugar cubes - not on the buttercream.

The problem is that RI simply WILL absorb moisture from air and the buttercream and nobody can tell you now how long it will be OK. There are to many variables - humidity of air, temperature, how dry and stable was your RI to begin with...