Easier to Google this than ask for software advice on a travel forum...

Are you aware that when you open an image with Preview (not too tricky, it requires that you find the image in Finder and then double-click on it), Preview has "Adjust Size" as one of the options on the Tools menu?

If you want a batch resizer, there are several, just Google "Mac image resizer", read some reviews, and pick the one that seems appropriate for what you are doing.