How do you restrict user image size upload inside the app?

Hello, I have 11 Imagesprites inside my app on which there sized to 70x70 pixels; in the app, the user is allowed to upload a different image to change each imagesprite, How then can I restrict the image upload to be exactly 70x70 pixels, and then an alert to let them know? Thank you, for any hints or advice...

Test the size of the image

size extension removed

Nope, I just re-read.

Use an image extension that provides the image height and width. There are a couple out there.