The piece of brass you found is not from a "picture frame." It is a piece of a civil war US Officer's "false embroidery" shoulder-insignia.
That style of metal insignia is called "false embroidery" because the stamped-brass metal is made to imitate the look of stitched thread embroidery. Yours still has some of the original goldplating on it.
The one in the photo below shows a Captain-rank version, which has a single border (or "frame") around the shoulder-strap insignia. The piece you found is from a double border frame, which was for ranks above Major.