The syntax is correct
knitr::include_graphics("../gladstone.jpg")
#> Error in knitr::include_graphics("../gladstone.jpg"): Cannot find the file(s): "../gladstone.jpg" # because `reprex`
# in console returns
# > knitr::include_graphics("../gladstone.jpg")
# [1] "../gladstone.jpg"
# attr(,"class")
# [1] "knit_image_paths" "knit_asis"
but you are not in the directory within the R session that is under ... Check with
getwd() # or
dir("..")