-In Instagram, go to the camera screen by tapping the little camera icon in the middle. Here you have the option to either take a picture or pull from your "archives". To do that, tap the icon that looks like two squares that overlap one another on the left hand-side.
-That will bring up the Photos App from your iPhone and you should see your albums. Tap on desired album (in my case "Camera Roll").
-Select your picture by tapping on it.
-A screen will show up on Instagram where you can "Move & Crop" your photo. Once you have the image how you want it, tap "Choose".
-The next screen you will see is one where you can pick the filter of your choice. Tap the green check mark when you're done.
-In the following screen, you have the option to add captions or share the photo. Tap your preferences, then tap "Done".
-And that's all! You should see your photo on your "Feed" screen.
I've played around with images that I have taken or that I have received via text or e-mail. Basically, you can do this with any image you can save on your Camera Roll.
It just dawned on me that if my 9-year old knows this, perhaps you do too. Hm, sometimes, my flightiness amazes even me! Well, here's to hoping that you didn't know and I've helped you. Cheers!