Superphil
Dismember
Is the right answer.
One of the two bits of software I miss from my Windo$e days.
Have you tried https://www.macupdate.com/app/mac/38381/fast-image-resizer
Another vote for irfanview, for a freebie It's an awesome bit of software (and very lightweight)
I used to use it, it has a lot of features, but for simple, no frills, resizing, http://imageresizer.codeplex.com/ is a featherweight.