You can use the RStudio addin to interactively select ggplot objects, or directly pass their names to the function.
Examples
# Shiny gadget
if (interactive()) {
ggplot_to_ppt()
# Or with an object's name
library(ggplot2)
p <- ggplot(iris) +
geom_point(aes(Sepal.Length, Sepal.Width))
ggplot_to_ppt("p")
}