New to Instagram?

I've been using Tumblr for quite some time now, and I've decided to try to make a popular Instagram account (10,000+ real followers), but I'm unsure of a couple of things:

Will I risk getting banned if I only post pictures I found on the internet to my account?

What would be a good niche if I want to get a lot of followers quickly? Would it be a good idea to make a fan blog dedicated to a popular TV series, a hobby or a musician?

How often should I post something?

You won't get banned as long as you credit the photographer. If the photographer is on IG it would be nice to credit their username, I see some people just crediting their name which is not right.
As far as niche is concerned, there are so many, most of the popular ones (fashion, travel, animals, fitness, humor) is way too saturated so find something different if you can, it will make you stand out from the crowd. Finally, as far as how often to post? A good number is 2x a day.

Thanks for the help. I have decided to specialize in sad quotes, because I noticed that it has the potential to gain a lot of followers and because my Tumblr specializes in black/white photography, so I could link the two accounts together and gain even more followers. It's also a really good niche because it's easy to make original content (for my first post, I just took a picture I found on Google and added a quote and a filter to it)

I posted my first image fifteen minutes ago and I got 10 likes, 7 followers and 1 comment from it so far and I haven't even followed anyone yet - is that good?

that sounds quite good and if I might say you chose quite an interesting niche, good luck with it.

There is always a risk of getting banned if you use images which you don't have rights to. However, the risk is very minimal imo.

As for picking a niche, you can look at some shoutout selling websites or even do a research yourself on Instagram to see which niches are the most popular .