A few things to note to increase your odds of getting your gallery posted:
Format screenshots to .JPG before sending
A gallery should have at least 10-12 screenshots
Vary screenshots (eg. If sending in a gallery from your Vex Thal raid, send in screenshots from your entire raid; don't limit them to one event or one mob)
If it's already been covered in another gallery, we're unlikely to post it (ie. we don't want another Halfling High Dive)