I had an issue like this recently as well. What solved it for me was
changing my logo's dimensions to 400x400. It looks like the pictures have to be that size or smaller in order to be loaded.
From what I understand, your logo is currently a .tga (good move!), though .png will work as well. Also, I don't know what's going on with this .mat file idea, but that's not necessary at all.
Also, make sure in your menu.cfg that you've added "MenuLogo = filepath" in the Main node (should go right under the Music filepath). Here's mine for reference:
<Main
MainFadeInTime = "1"
MainFadeOutTimeFast = "0.3"
MainFadeOutTimeSlow = "3"
TopMenuFadeInTime = "1"
TopMenuFadeOutTime = "0.3"
TopMenuFontRelativeSize = "0.05 0.05"
TopMenuStartRelativePos = "0.8 0.325"
TopMenuStartRelativePosInGame = "0.5 0.5"
TopMenuFont = "menu.fnt"
MainMenuLogoStartRelativePos = "0.0525 0.22"
MainMenuLogoRelativeSize = "0.47 0.47"
BGScene = "Amontillado/maps/menu_bg.map"
BGCamera_FOV = "70"
BGCamera_ZoomedFOV = "40"
ZoomSound = "ui_zoom.ogg"
Music = "../Amontillado/extrasounds/Kosmogonia_Intro.ogg"
MenuLogo = "Amontillado_logo.png"
/>