You can also do this by getting a Dropbox account, adding it to your pc and phone, tablet etc. Then put the photos you take with your camera in your Dropbox folder on your pc, they will show up in the dropbox folder on your phone too, then when in Instagram you can select to use a picture from the dropbox account. My phone camera's not that great either so that's what I do, except when I take a work in progress shot and then I just use my phone to take a quick picture.
I've been using dropbox for a few years and love it. It means I have all my photos handy on my phone in case I'm out and about and want to show somebody, like a portfolio I also keep my accounts files stored on it too in case anything happens to my computer.
Hope this helps x