Hi,
Sounds like you have an older version of the Mexico WMS access string.
Here's one that works:
/cgi-bin/map4/mapserv_orto?LAYERS=Mexorto&MAP=%2Fwebd%2Fmap%2Fweb%2Forto_wms.map&SERVICE=WMS&SRS=EPSG:4326&VERSION=1.1.1&FORMAT=image/jpeg
Likely you'll just need to change the end from gif to jpeg. The rest, I am pretty sure, should be the same from your older version of TF.
Let me know if that doesn't fix it for you.