To get a new image to start a tag with you can go to file to New Image
or you can also click the little icon that looks like a white paper for a new image.
You will get a pop up window example below:
You see in this example I have 400 by 400 and
I have white for the color in the middle.
If you wanted transparent you would just put a
check by the box that says transparent.
We have the background raster dotted
and 16 million colors in the color depth box.
These are what we normally use.
Basically most times you will just need to put the size you want,
and the transparent or not transparent and click ok.