Jetpack Joyride

No sharing is actually involved for this achievement. When you finish a run of the game and are on the after-action screen, click the small picture of you playing on the left-hand side of the screen (under the display of your distance). This will take you to that image. Now you can click in the bottom right side of the screen on the picture of a floppy disk to save it to your phone. As soon as the pop-up that says "Saved to Camera Roll" comes on-screen, the achievement will unlock.

After you have died and completed your run, you will be presented with a Summary screen. There will be a picture of you during your run. Click it and click the share button and the achievement will pop immediately. (You do not have to actually share the shot.)