How about using Texture2D.LoadImage? It returns a boolean. The documentation doesn’t say anything about it, but i guess it will tell you success / failure of the function.
private IEnumerator LoadImages (string path, UnityAction<Texture2D> textureCallback)
{
WWW www = new WWW (path);
yield return www;
if (www.error != null) {
//there was an error
textureCallback.Invoke(null);
} else if (www.texture.width == 8 && www.texture.height == 8) {
//there seems to be an unsupportet file or the user really passed a 8 x 8 image.
textureCallback.Invoke(null);
} else {
//all fine
textureCallback.Invoke(www.texture);
}
}