You're actually going to end up editing up to 6 different image files depending on what you're trying to accomplish.

On your system, go ahead and navigate to C:\Program Files\Stardock Games\Sins of a Solar Empire\Textures (or where you installed the game).

In that folder you will find many TGA and DDS files. You're looking for:

Unit_Hud_Pressed.tga
Unit_Hud_Normal.tga
Unit_Hud_Disabled.tga
Unit_Hud_CursorOver.tga
Unit_Picture.tga
Unit_Main.tga

The first four correspond to the bottom right menu buttons in the user interface of the game when a frigate/capital ship factory is selected. Each image file has a specific label which is pretty self explanatory. 'Pressed' is when the icon is clicked, 'Normal' is the default icon, 'Disabled' is when it cannot be constructed and is therefore grayed out, and 'CursorOver' is when your cursor is over the icon.

Unit_Picture.tga is the main picture that is displayed in the bottom center of the screen when a unit is selected.

Unit_Main.tga is the overlay picture that replaces your ship when zoomed out over a planet.

As far as I know paint doesn't support TGA or DDS files by default. You'll have to use a separate program (Photoshop, Paint Shop Pro...) to edit these files. Make sure you back up the originals, or just create your own mod and use the edited files there.

Sorry it looks like I missed a file.

Unit_Infocard.tga is another image file with those little icons you're looking for. They seem to be a slightly smaller version.


*digs around a little more to make sure there isn't any more...*

What file formats can Fireworks Import and Export?
Fireworks imports:

AI
BMP
Fireworks PNG
FreeHand FH7-11
GIF
Illustrator 8
JPEG
JPEG 2000*
MacPaint*
Photoshop PSD
PICT (Macintosh only)
PNG
QuickTime Image*
SGI*
TGA
TIFF
Fireworks exports:

BMP 8, 24 bit
CSS layers
GIF and animated GIF
HTML
Illustrator 7 (.ai)
JPEG
Macromedia Flash (.swf)
Photoshop PSD (.psd)
PICT 8, 24 bit
PNG 8, 24, 32 bit
TIFF 8, 24, 32 bit
WBMP
*QuickTime plug-in required

DDS is supported with a plug-in I believe.