You have to use a 3rd party site, but one I use frequently imgur.com does not have a mobile app. Here’s what I do: I select the pics on my phone and email them to myself. I go to my desktop computer and get the email and save the attached pics to the pictures folder. On the desktop, I go to imgur.com and upload the pics from my desktop. From there you just select resize to 640x480 go into each pic and copy/paste the img code. Sounds hard, but once you do it a couple times, it’s really easy. You learn to open 2 tabs, one for here and one for imgur.com and go between the two copying and pasting.
If you do not have access to a desktop ignore the above