Hi oxbob, welcome to the forums. We are a helpful bunch, and I hope you will enjoy your time with us. Now back to your question. I don't have an iPad mini, but I expect the apps with both iPad and the mini work the same.
I'm guessing that you would like to use a photo for your Avatar. I change my avatar from the iPad forum app. I attach a screen shot which shows the stage at which you are ready to use a photo as your avatar. To get here, tap the profile button at the bottom of the screen. Tap inside the image box (your's is a grey and White silhouette at present). Chose insert from photo library in the prompt box. Pick the photo you wish to use and confirm. Go back to the posts page. That's it..
If you wish to update your avatar from the forums.net site, then go to your details. Tap settings in the strip menu towards the top of the window. The, select change avatar from the sub menu and follow the steps.
I hope this helps.
Sent from my iPad 1 using iPF - Greetings