Ah, but this is the best place to ask such a thing

When my husband went to take his picture for a Canadian passport, it was about twice the size of immigration photos. I guess that's why they write that warning there, because passport pictures in Canada are a lot different.
When I went to get my picture taken for immigration, it was exactly the same as I would need for a Slovenian passport, except the background had to be white instead of the traditional blue. The background on mine turned out white though, not grey.
If it's a little grayish, I think it should be ok. If it's gray, it won't. If you send them and they won't be ok, one of the two will happen: Mississauga will return your app to you and ask for new pictures OR the visa office will send you a request for more photos. So it's not a disaster that will get you rejected, but it would delay your application.