I can't load an image (Mac/OSX)
QuartzImageIO should be fairly easy to install on OSX, and is the recommended first choice on macs. Unfortunately, it does not work for all image types.
ImageMagick is more broadly-capable, but is also finicky particularly for mac users. See https://github.com/JuliaIO/ImageMagick.jl#osx for some things to try.
I can't load an image (Windows and Linux)
These platforms use ImageMagick by default. The first thing to try is
Pkg.build("ImageMagick"); if you see any errors, these are likely to be the cause of your problem. See the troubleshooting section of that page for more help.
I can't display an image (ImageView)
ImageView depends on Gtk; if the error messages suggest this may be the problem, see Gtk's troubleshooting page. You can test your installation of Gtk with
Pkg.test("Gtk"); if it passes, the source of trouble is likely to lie elsewhere.