Supid Question about IG?

I have 3 IG accounts with real 500 followers each, for example in one of my accounts I'm targeting Dog lovers niche, So i post 5 posts everyday "dog photos" and after 3 days i post a Teespring t-shirt,
NO SALES UNTIL NOW
do you think what i'm doing is wrong ?
I mean should i just post t-shirts not dog Pics?, I'm new at IG so just a small advice from you guys will be so helpful for me, Thanks in advance!