You replacement TGA has to have the same direction and size as the one you are replacing.
The textures are mapped using UVW mapping inside 3dsmax/gmax. Which doesn't care about the actual image used, but DOES care about it's size and rotation.
Likely, although I do not know this for fact, but likely the original Bioware TGA files had more than one shield on them. So, the 3ds editor picked which part of the texture to map onto the shield. Also likely, the Shield TGA used by Bioware had multiple parts of the shield inside that same texture, the handle bits, the rims, the actual face of the shield etc...
So, a standard "picture" of a shield won't work, you need one that has the same parts/pieces and be the same size (at least same power of 2 IE 64k, 128k, 256k etc). Otherwise, you will have to modify the actual shield mdl file, and manually remap the texture that you wish to use... This should NOT be necessary though, as long are your new texture meets the same dimensions and parts as the original.