24 bits bmp != 256 colors bmp. 8 bits images = 256 colors (color table).
In RO, we don't create acts, we edit them. We use official RO acts to make our own acts (by editing them, in case it's needed). Use an official act that suits your sprite. Change it's name so it will have the same name your sprite has.
You can edit the position by modyfing it in the same frame, instead of changing Xoffs and Yoffs values in the act (since it would take much more time).
If you have to make other edits such as number of frames, opacity, rotation, etc. modify the act with ActOr 1 and/or 2.