I am a recent convert to the Mac, very pleased! My PC had a Microsoft "Image Resizer" that was a free download from their site that made high rez images smaller for email and the like. Does such an application exist for the Mac?
Hello - you've already been instructed to use Preview which is a powerful OS X built-in app already available for your use - take a look at this Apple Support Article for a review of some of its features.
BUT, just to get you started w/ some visual aids - first image below is of my den bar opened in Preview - the Toolbox (or Markup) icon (blue arrow) will open the menu bar below; the red arrow points to the 're-size' option which is opened in the second image - there size can be easily adjusted. Also, note that there are many other choices in the menu bar in the first image - that's how I added the arrows and changed their colors. Explore this utility and you will likely not need much else for your basic image edits; also a great PDF reader - Dave