Instagram

From what I see it is subjective and photography itself. Personally I only post from the IG camera as marketing to my stuff on InstaPrints .... then I find followers click my bio links to here.

I personally would never think to use DSLR images in IG. First, I think you need a 3rd party app to do that.

..... but I tend to be more of a 'purist'.

Considering the number of people posting with s120's, RX100's, A6000's, 6D and 5D Mk II's, D610 and D800's etc etc, I don't think it matters - just use whatever you feel comfortable with.

I share the images from my samsung phone from for example my flickr site..I rarely take a pic from phone anymore,..Glad to know that others are doing so too..

I post all photos from my camera. As sort of a marketing standpoint. AW link in my bio, etc. I would say for sure a yes. There are tons of pros out there that post their gallery on IG from camera photographs.

IG is actually proud of the fact it is for Smartphone photos only. I think some people may feel you are cheating if using a camera instead

I do and when I do I tag it as such. No 3rd party app is needed. Just edit any photo into the square format, copy it onto your phone and use the IG app to place the photo onto IG.

If you wish to keep the same ratios then yes a 3rd party app is needed but the one I use is free InstaSize.