Xeffer, are you asking how to add transparency to a png, or are you suggesting there's something special to transparency in a guild emblem?
I'll assume you're asking the former. Since this is dependent on the tool you're using, let's assume you're using Pixlr (http://apps.pixlr.com/editor/). Let's also assume everything Eagle posted above is true.
- From the load screen, select "Create a new image"
- On the New image dialog, change Name: to "guild" (no quotes), and change Height and Width to 48.
- Check the box labeled Transparent, then press OK
You now have a 48x48 transparent layer to work with. The first thing you want to do is go over to the Layers panel on the far right, Right-Click on Layer 0, and select Duplicate Layer. This will give you a new layer named Layer 0 copy. This is the layer you'll want to draw your picture on. If you're not used to working with layered image editing tools, play around with this a while and get used to how the layers and the transparency work.
Once you're done with your image, select File > Save, make sure you're on My Computer, and change Format to PNG.
- If you see the small gray and white checked pattern, that area is transparent. If you see solid white with no checks, that area is white. Use the eraser to remove it, and it should become transparent (at least, on that layer).
- Layers stack on one another. Whichever layer is at the top of the list in the panel will draw over all layers below it. You an test this by drawing a red line on one layer and blue line on the other to form an X. Move the layers up and down in the list to see how the overlay changes.
- Whichever Layer you have selected in the list is the one you're editing. If you're trying to erase something and it isn't going away, you probably have the wrong Layer selected.