I found official documentation about naming convention:
LINK
Here it is an excerpt:
ppp_oooooo_tttt_ffff_c.IMG
ppp = Mission Phase:
INT = Integration and Testing
CAL = Calibration Observations
ATL = ATLO Observations
KSC = Kennedy Space Center Observations
SVT = Sequence Verification Test
LAU = Launch
CRU = Cruise Observations
APR = Mars Approach Observations
AEB = Aerobraking Phase
TRA = Transition Phase
PSP = Primary Science Orbit (nov 2006-nov 2008)
REL = Relay phase
E01 = 1st Extended Mission Phase if needed
Exx = Additional extended Missions if needed
oooooo = MRO orbit number
tttt = Target code.
The target code refers to the latitudinal
position of the center of the planned
observation relative to the start of orbit.
The start of orbit is located at the equator
on the descending side (night side) of the
orbit. A target code of 0000 refers to the
start of orbit. The target code increases in
value along the orbit track ranging from 0000
to 3595. This convention allows the file name
ordering to be time sequential. The first
three digits refers to the number of whole
degrees from the start of orbit, the fourth
digit refers to the fractional degrees rounded
to the nearest 0.5 degrees. Values greater
that 3595 identify observations as off-Mars or
special observations.
Examples of target code:
0000 – planned observation at the equator on
descending side of orbit.
0900 – planned observation at the south pole.
1800 – planned observation at the equator on
the ascending side (day side) of the orbit.
2700 – planned observation at the north pole.
Off-Mars and Special Observations Values:
4000 – Star Observation
4001 – Phobos Observation
4002 – Deimos Observation
4003 – Special Calibration Observation
ffff Filter/CCD designation:
RED0-RED9 - Red filter CCDs
IR10-IR11 – Near-Infrared filter CCDs
BG12-BG13 – Blue-Green filter CCDs
C Channel number of CCD (0 or 1)
![Image](http://www.planetmobile.it/jumpjack/immagini/marte-hirise-nomi.JPG)