To install Brew's ImageMagick and fix its Symlink
A list of things to try, in order (I'm writing an answer instead of a comment for better formatting). Run hash -r after each step (assuming you're in bash; do rehash instead for zsh; or you can always open a new shell).


Run brew --prefix and see if the output is /usr/local. If not, your brew is not installed to /usr/local! Add $(brew --prefix)/bin to you $PATH.
Run brew info imagemagick and make sure it's actually installed; maybe you had an error during installation and it was simply not installed successfully.
Run brew link imagemagick to symlink to /usr/local/bin; maybe the executables weren't symlinked correctly (for some reason);
Run brew reinstall imagemagick to reinstall.
If it still doesn't work, show us the output of brew list imagemagick and we'll see what's wrong.