@kina, I heartily and totally recommend Affinity for a Photoshop alternative (much, much more inexpensive!), I don’t know that I’d recommend Darktable or RawTherapee as a Lightroom alternative/Raw developer (complex and hard to use, in my opinion. I’d recommend Skylum’s Luminar, which is around $69 right now).
However, if you have a Mac, the simplest (and cheapest) way to change a photo’s resolution is probably with Preview (and it’s already on your computer). It’s the default application to open a JPEG file (or just about any image file) when you purchase your computer, but sometimes other applications will set the default to themselves.
If your image is sitting on your desktop or in a folder somewhere (not in Photos), fantastic. That makes it easy. To open an image with Preview from your desktop, you’ll want to hold down the control button on your keyboard and click once on the image file. You’ll get a little pop-up menu, and you’ll want to hover over “Open with” until it displays a list of applications. Preview, if it’s the default application, will show up at the top of that list. If it’s not the default, it’ll be in the list of applications.
If your image is in Apple Photos and you haven’t done any edits or cropping to it, you’ll want to “export the unmodified original” to your desktop. The easiest way to do this, I’ve found, is to hold down the option key and drag the image to your desktop. The other way to do it is to click File –> Export > Export Unmodified Original. You’ll get a pop-up menu (I think you could probably leave the IPTC as XMP unchecked (I think it stores any edits in that XMP file so you can open it up in something like Lightroom, but I’m not sure)).
If your image is in Apple Photos and you have done some editing to it, you’ll want to click File –> Export > Export Photo. In the pop-up menu, make sure you set the JPEG quality to Maximum and the size to Full size (as you’ll be resizing the image in Preview).
What you don’t really want to do is just drag the image out of Photos onto your desktop. This, by default, downsizes the photo, and you want to do all of your downsizing all at once in Preview.
After you’ve exported the image out of Photos, find where you saved it, and then go ahead and resize and change the resolution with Preview as described earlier.
I hope this helps out and isn’t too confusing! Good luck with changing the resolution!
—@loganlamar