How do I tell how many pixels an iPhone photo is and can I change it to a jpeg format?

I'm entering a photo contest and the entered photo needs to be so many pixels and in jpeg format.

Transfer the image to a computer.

The image's metadata includes the width x height pixel count.

Photos captured by an iPhone are JPEG files.

First, upload the image to a computer.
Next, right click on the image and choose properties. There you'll be able to see the resolution.

Or, you can lookup the specifications for you camera in your phone and check the resolution there.

Next, you'll need to open the image using some kind of image editing program. Grimp is free at gimp.org.

With gimp, or which ever app you use, then change the resolution to the required size. This is often achieved by changing the canvas size of the image. The canvas size is the H & W of the image file itself. Next you'll need to save the new image, but Gimp doesn't have a "save" option. Instead of "saving" the file, you'll need to choose "Export" which allows you to save the file as a new JPEG. NEVER overwrite the original image. When you export the file, you must choose a new file name or else you'll loose the original file.

Open the photo in a graphics program and see properties or you can attempt to resize it. Without actually doing the resize, multiply the length and width that come up. That's the total number of pixels, granting the program measures in pixels. To get megapixels, divide the result by 1 million.