A 24-year-old D.C. Jail inmate was charged with homicide yesterday in the fatal shooting near Dupont Circle last spring of a 30-year-old French woman on her first trip to Washington, and in the subsequent slaying of a photography store operator in his studio at 2025 I St. NW.

Police said they believed robbery was the motive in both of the fatal shootings.

The suspect was identified as Robert Eugene Swann, formerly of the 3400 block of 14th Street NW. Authorities at the jail said he began serving a 3-to-12-year sentence there on Oct. 21 after pleading guilty to three counts of burglary.

Jail records also showed Swann had been paroled Nov. 25, 1981, from a sentence for petty larceny and unlawful entry.

Investigators declined yesterday to tell what led them to charge Swann in the two slayings. He was taken yesterday from the jail to D.C. police head-quarters and, after questioning, was charged in the two deaths, police said.

In the first slaying, which occurred about 3:20 a.m. on April 10, Chantal Mouyal, a physiotherapist here on vacation from her home in Paris, was shot twice near 19th Street and Sunderland Place NW after three robbers forced her companion from his car, robbed him of $60 and locked him in the car's trunk, police said.

A friend with whom she was staying said she had asked about the safety of the city.

In the second incident, police said Robert J. Tobin, who operated the Creative Photography studio on the first floor of a building at the inter-section of Pennsylvania Avenue, I and 21st streets, was shot about 1 p.m. on May 27. Police said at the time they were seeking two assailants.

Police said yesterday that Swann said he was innocent of the homicide charges.

In a brief interview yesterday, Swann said he had attended Coolidge High School here through the 11th grade and had subsequently worked in construction and as a mail clerk. He told a reporter that before being sent to jail he was using co-caine at a cost of $150 a day.