Look in the screenshots folder (located in C:/Program Files/Blockland or wherever you installed the game) for the right one and then attach it to your post using the menu at the bottom.
Use a background of Space-printed identical colour glow print blocks for a uniform background it is easy for card-makers to cut you out of.
Try using the mega shot and then resizing the picture for better quality - then it looks better on the card and won't look bad if you reduce the size so it fits within the forum's limits. (512kb for both attachments, two largest selectable resolution non-mega screenshots go over this)
