Okay, on a Mac, I think this is the simplest way to accomplish what you need to do, i.e., without learning anything too complicated, without installing extra software, etc.:
1) Open the pic in Preview. You can just drop the pic onto the Preview icon at the bottom, or run Preview, and do a File --> Open.
2) Click View --> Show Markup Toolbar
3) Click the Resize icon -- I'll show this in a second.
I just did this, starting with an iPhone pic that I took at the end of a day-trip yesterday. I had downloaded that pic to my Mac, and it had dimensions of 4032 x 3024, with a size of 2.9 MB. That the sort of pic that this forum considers too large to put in posts.
Okay, having done steps 1 - 3, this is what Preview looked like, and I have added an arrow, to show you the Resize icon:
View attachment 3468
4) Click that resize icon, and you'll get a box for the new size. That box starts out with Inches as dimensions, but that's just too weird. There's a drop-down box -- I've indicated this by another arrow -- that I suggest you change to Pixels, as I've just done:
View attachment 3469
5) Over-type that 4032 with, for instance, 1000. Then click OK.
6) Click Save, or Save As, if you want to preserve your original file. (You have to hold down the Option key to get that Save As to show up as a choice under File.)
Those steps will change that original 4032 x 3024 to a pic of 1000 x 750, and from a size of 2.9 MB to approx 200 KB, i.e., a reduction of size by more than 90%. Those 1000-ish pixel files are detailed enough for viewing on forums, and the 200- or 300-ish KB size is acceptable here. I don't know what the upper limit of pic KB or MB sizes are here on the forum, but I've never had, say, a 200 KB file rejected, but have had the