I am not an expert when it comes to MacBook shortcuts and tricks, I am still learning how to master it :D
I used to always resize my images using a software in Windows 7, but however, since I have shifted to Mac I had to look for a solution to resize my images while keeping the high resolution option valid. The following steps will show you a very simple and easy way to resize your images without the necessity of having a third party app.
I know most of you know this, but I hope this might be helpful for someone else out there :)
PS: Make sure you have your original photos saved in another folder, as the following steps will be replacing the original ones. Unless you duplicated your resized image.
STEP 1: Open your photo
STEP 2: Go to File > Adjust size
STEP 3: Adjust the resolution
As you can see the size of this photo is 1.1 MB, and after I have adjusted the resolution it became 248 KB.
The photo will look like this, but don't worry it will still have the high resolution option, enlarge it and see it yourself.
STEP 4: Save your resized photo
You have two options, whether to save it on the existing photo or duplicate the photo and save it as a different one.
Congratulations! Now you have your resized photo ready to be shared :)
Hope this was helpful somehow :)