Complex sentence: It is not know if Private Howard had previously trained as a photographer or if he acquired his skills after arriving at Fort Bridger, but by the summer of 1876, he was actively producing photographs of the post and the surrounding area.
Split into simpler sentences: It is not know if Howard had previously trained as a photographer or if he acquired his skills after arriving at the post. But by the summer of 1876, he was actively producing photographs of Fort Bridger and the surrounding area.