Here is a link to the roster of Company E, which lists Mr. Sweeney. http://civilwar.illinoisgenweb.org/r100/051-e-in.html
You mention his possible wounding @ Chickamauga, Georgia in Oct. 1862. This battle actually occurred Sept.19-20, 1863, nearly one year after he had been discharged from the 51st Illinois.
On Ancestry.com, Augustus appears on the U.S. Civil War Pension Index. He filed as an invalid on June 10,1866. (Application No. 111501; Certificate No. 855714). The National Archives might have additional documentation pertaining to his application, possibly providing more detailed information on his service.
As for a possible photo, you'd better check with living descendants and others researching this family to see if such has survived.
Greg C. White