If you save a pic you find online to your iPhone’s photo album or want to select a photo you have taken, you can resize it. Save a pic to your photo album. Then, go back to your post and click attach files. Choose your photo library from the choices. You will be taken to your photo album. Tap on the pic you want to attach and a green check mark appears. At the bottom of the page you will be asked if you want the actual size or if you want to resize it. If resizing, you can select from Large, Medium and Small. If they are all too big, you can crop the pic to shrink it’s size by clicking on edit. Once done, try attaching it again.