The adverts are in the Reiza Common Folder, which is located under "Locations". You have to open the mas files with a mas editor or extractor. Easy to find, just google for gmotor 2.0 mas utility. It is just an exe. Then you can extract the adds.dds - files, work on them in Photoshop or an other painting software, reload them to the mas and create a new one. No big thing.
Some add on - tracks like Adelaide have their adverts within their own locations folder. You have to open the textures.mas - file there.
Did not play AMS online often, but never had problems with mismatches, because when you just alter a . dds file, the game does not care about it.
Be sure to backup the original files, in case that something goes wrong.
After packing a new mas, the program tends to crash on my system. If this happens, don't panic, just close it and reopen. Will instantly work again with the next file.
Have fun.