Air on iPad/Android tablet, overwrite partial image

[Frilli7]

(For the busy people)
In AIR on iPad/Android tablet is there a way so I can open an image (not display it), overwrite part of that image with new bitmapdata and save a new instance of that image so that it goes to the PhotoStream/Gallery.

(For those who have a minute)
So I'm making an AIR app for both the iPad and Android Tablets. It's basically like those avatar builders where you can choose a body, arms, legs and all that stuff and make your own custom character. The app is actually almost done. The user can make his own character, share it to his facebook page, enjoy some great AdSense ads and listen to some original music while doing so. Also I added a feature that lets the user save the image he created (the character has optionally a solid color background or an image background) to the Gallery(Android) or Photos(iPad). So basically everything works... except for one little thing that's bugging me.

I use the JPGEncoder and just grab that part of the stage (frame) where the character is and save that image with FileStream. Really simple stuff. However the image isn't very big, it's only 210x400px and both the Android tablet I'm testing on (Galaxy Tab) and the iPad scale up all images when the user is browsing through them. That means that all the character images created from my app are pixelated, i.e. the quality is just not good enough. I've even had kids test the app and they're usually pretty disappointed when they see the images they saved because they were really crisp and nice inside the app.

So I got this idea but I don't know if it's possible... could I have a jpg/png image that is 210x"device's screen height". When the user wanted to save his character, the app would open that image in the background, grab the pixels on stage (the character), save those pixels onto a part of the big image and then save the whole thing to the Gallery/Photos.

That way I figured I could place the company's logo for example on the bottom and stuff like that. It would be like a template.