what you do is, see which folder the acog belongs in open pakscape then navigate through your program files find mw2 folder, >main>(search for all types) then click on iw_0x.iwd>images>paste in what you want
Inside will be a bunch of iw_xx folders
Open the iw_xx that matches the one in the mod you dl'd with Pakscape (or winrar/7zip)
Copy the iwi's from the mod you dl'd into the images folder in the iw_xx dir in your main folder
MAKE SURE it asks you to overwrite. If it doesnt you are probably putting it in the wrong iw_xx
Save and play.
/simple instructions. I've had to explain this to many people, many times.