I don’t normally use TGATool2 tool this way but I followed Michael’s instructions on the first page of this thread to test and it worked for me.
I found I could mangle the exported ACE file if I resized the image in MS Paint. In MS Paint there are 8 boxes around the edges of the image. Do not move these boxes as they resize the image. Textures for MSTS must be square and 32x32, 64x64, 128x128, 256x256, 512x512 or 1024x1024 pixels in size.
To recap the steps are:
1. Make a backup of the ACE file you are about to modify.
2. Open the ACE file with TGATool2
3. Use the following menus in TGATool2: Image->Send Main Image to Editor.
If you are using MS Paint as your image editor then MS Paint will automatically load and the ACE file you selected will now be converted to a BMP file. You can edit as desired using the MS Paint tools. Use File->Save if you wish to save your changes as you are progressing through the ‘painting’. You may be asked if you wish to save the file. Click on Yes.
4. When finished making the changes you desire use the following menus in MS Paint: File->Exit. You may be asked if you wish to save the file. Click on Yes.
5. TGATool2 will now popup with the image you have just edited.
6. In TGATool2 use the following menus: Export->Ace (No Alpha) if your texture has no Alpha channel, or Export->Ace (Alpha) or Export->Ace (-Trans) if your texture has an 8 bit or 1 bit Alpha channel respectively. Here you get the option to change the name of the ACE file if you wish.
7. Test using ShapeViewer or MSTS.
As Melbournesparks has advised please provide as much information as possible as it makes it a lot easier to assist you.