Update: The Blizzard Store has been updated with four new posters for the Worldwide Invitational including a poster of Kerrigan and one of the main villains from all three franchises:

Day 2's splash screen has been released:

Not only has the image transformed but a new symbol has become clear on the right side. It is quite obviously a
Hel rune from Diablo II:

The one on the left now seems to resemble a
Shael rune from Diablo II as well. It appears as though the Diablo rumors/theories were correct. Here's the second part of the weird purple puzzle piece that was released today - it was titled: 15.jpg (also named "cavern in the CSS file"):

If we continue piecing the puzzle together (according to DiabloFans' theory from yesterday) we'll get the following image:

Arranging it horizontally also seems valid:

Also notice that the labels for the images ("Tundra","Cavern") are similar to places from Diablo II's expansion: Lord of Destruction: The Frozen Tundra and The Drifter Cavern (in the Glacial Trail).
SC:L will be working around the clock to keep you updated!