How can I visualize Git Flow branches?

You have not specified whether you are looking for a command line tool or not, but if so, I find that something like this gives you a good appreciation of the branch structure

git log --all --color --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit

And of course you can create an alias for this with git config, eg

git config --global alias.lg "log --all --color --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit"

If your code is in a public github repo, you might want to look at http://beta.gitflowchart.com. For Atlassian Stash, there is a plugin that does this too (not free).

Disclosure: I wrote the underlying library for both tools.

I suggest you to use GitKraken to get such images.
It is a Git GUI client for Windows, Mac and Linux.
In it you have a nice visuals like the Commit History.

There are more Git GUIs, such as:

• SourceTree, available for free for Windows and MacOS.

• Tower

Here's a list of them from Git's official website and a list of them from Wikipedia that may be of help.