Is there anywhere to get Blank Icons? (for Guild rank Insignia, Quests,etc.)

Also what folder are the rank icons in?

http://cs.elderscrolls.com/constwiki/index.php/Category:Factions a whole lot about location.

As for "blank" ones, can you define how much standard content you expect on a "blank" one? Otherwise, just telling your image software to create a new 128x128 image will give you a totally blank one, but I suspect that's too blank! You can probably create a starting image by loading up an existing one, selecting the background color from it, and just clearing the center to that, but if the insignia icons need a common background image, there's no easy way to get back the layers from the artist's original.

(Edit)
The Knights of the Thorn insignia has a lot of background showing, and that same background was used in a lot of other icons. If you copy the lower right and flip it to the other corners, you'll probably get a reasonable background for making your own. The general method Bethesda's artists seem to have used is to create the central design on a transparent layer, and then copy the layer a couple of times, growing the outline each time and flat-filling one with the background base color and the other with the background detail color. These are placed under the main detail to create the borders. The hard part will be getting a consistent style to go with the existing ones, but there are a lot of details that appear to be copied from one icon to another, such as the arms on the fighter's guild ones, and the ribbons under the designs on others. These parts can easily be cut-and-pasted into your own.