Could we also say "La foto tre bonas"?
Not quite. That would be "The photo is very good." However, you could say "La foto tre belas."
Yes. Yes it is.
In times past "fotobildo" would indicate a photographic picture.
I tried to use "bildo" - is there much of a distinction?
A "bildo" can describe a photograph but it's more general, so it may also refer to a drawing or an image in your head. Conversely, "foto" (or "fotografaĵo") is specific to photography.
En la hispana: la foto es muy bella/bonita
En la portugala: a foto é muito bela/bonita