OK. According to their website you get 5MB of webspace (assuming you are on high-speed/DSL and not plain old dialup).

That is more than enough for you at this stage.

Your Internet Service Provider (ISP).

Whoever you pay to let you connect to the Internet usually (not always) allows you some space to upload your files to.

Who is your ISP?

Your ISP most probably provides you with some free webspace. Most do.

In theory there's no reason why you can't use them, although some users may have compatibility issues with their browsers. This link is up to date:

http://www.libpng.org/pub/png/pngapbr.html
...

If you're going to be serious about it, though, you might want to consider buying a domain name and webspace to go with it. You often get what you pay for, and if you pay nothing....

I'm not...

How secure does it need to be?

The problem with Javascript is that the code is in the page, so it is quite easy to crack it.

You'd be better off using a server-side script (PHP, Perl, etc.)....