you actually arent guaranteed the largest resolution that way
to be sure you are getting the top resolution IG offers just delete the resolution part of the image path
the part of the URL you are looking for is in this format:
[SNIP] 1.2885-15/s480x480/e15/fr/19 [SNIP]
just delete the "s480x480/"
so that part becomes simply:
[SNIP] 1.2885-15/e15/fr/19 [SNIP]
then IG will load the original image for you