There are a number of software items you can use for the purpose. I use Imagewell, which used to be free (may still be for all I know). Irfanview is another. I guess if you Google pic re-sizing or something like that you should find something.
Using Imagewell, you first put the pic on your computer. Then click on the Imagewell icon which brings it up on your page. Go to the pic you want to use and it's a simple drag and drop into Imagewell. There's a slider on the side of the Imagewell box and you simply slide it to the size you want your pic to be. I set mine at 900 on the longest side, whether it is a vertical or horizontal pic. Then you "save as"...now it is the size you want on your computer.
At the site here, when you have your post written out, go to the attachments below and click on. Hit the "browse" button, then go to your pic on your computer and load it. You're done. If you want to load more than one pic at a time, go back to the attachments after your first pic is loaded and click on the more attachments button and repeat the above procedure. Up to four pics at a time per post if you wish. If I haven't lost you by this time go to the head of the class! Good luck.