Images are located in:
X:\Program Files (x86)\Steam\SteamApps\common\Stellaris\flags\.

***Follow file structure on Wiki for beginning to mod****

Backgrounds are in the backgrounds fodler.
Other folders are for the logo designs.

Inside for each of these will be map and small folders.

Map is what you see on galaxy view. The white logo outlines.
Small is what you see on your planets to the left.
In Root is what you select in menu and also seen on portraits ingame.

All images are in DDS format. RGBA8, No Mips, No Compression, Single layer.

In map folder images are 256x256 px. Full Colour is allowed.
In small folder images are 24x24 px. Full Colour is allowed.
In root i.e .\pirate\ images are 128x128 px. Full Colour is allowed.

Name follow flag_<folder>_#.dds

You will have an image for each logo one in each folder.

.\pirate\ <Colour 128 px>
.\pirate\map\ <White 256 px>
.\pirate\small\ <Colour 24 px>

With name for example "flag_pirate_35.dds"

Make sure you put the right image in each folder as each image should have same name.

You can make your own folder like I have done, 6ft6. then my file names will be "flag_6ft6_1.dds"

If you do make your own folder in the menu you will have the default FOLDER_NAME_6ft6 unless you change the localization.txt files.

---------------------BACKGROUNDS--------------------------------

These are 256x256 images.

Can be named <anything>.dds

I used "6ft6spiral1.dds" using my 6ft6 prefix just incase other people have modes for a spiral1.dds too so we dont conflict or overwrite eachothers mods if people are using both of ours.

The Backgrounds use two colours Full Red and Full Green.

You can gradient between these two colours if you wish, but as long as Red and Green values are used only.

Save these in same dds format as the foreground logo. RGBA8, No Mips, No Compression. Single Layer.