_Seth_ is right, your emblems can be 64x64, 128x128, 256x256 etc. Find yourself an image that you want to use as an emblem, if you use google then they will most likely be jpeg.
Open this image in PS and remove the background so that only the detail you want left is visable on a separate layer.
Then create a new document with one of the above resolutions, it's best to keep this as near to your new image res as possible.
Fill this new document with Black or Dk Brown, SH3 uses these colours for transparency. Pick a fill colour that is different to the main body of your image.(i.e if your image is a black diamond then fill with brown).
Then drag your layered image onto your newly created filled layer. Center it up etc.
Then save this as a TGA at (IIRC) 24bit. You can then drop this into your emblems folder.
__________________
|