You can either email a pic to yourself,or I just take a screenshot of a pic on my phone. This automatically resizes it. No need to go find some APP to do it.
I'm barely computer literate so can't give you all the reasons why. But the above works for me.
Then,hit the attach files box,just below the message area when you post. That "should" take you to your phones gallery. Using the screenshot file.... I just click on the pic. This site,then starts uploading. Some sights you have another step or two before it uploads.
Once it gets to 100% loaded(you should see the % numbers increasing) you hit the "insert file" and it will ask full size or thumbnail...... this is from memory. I've done it so much it's second nature. And believe this,I'm an old codger that barely knows how to turn a computer on.