One of the uses of Roman coinage was to spread news and propaganda throughout the Empire. The Romans put out a lot of coinage types that celebrated their victories. Often these coins would depict some symbol that would be easily recognized (at the time) as being representative of that conquered land.

Far and away the most famous conquest coin are the JUDAEA CAPTA types struck under Vespasian and then his sons Titus and Domitian and commemorating the defeat of the First Jewish Revolt in AD 70.

http://www.wildwinds.com/coins/ric/vespasian/RIC_0159,note.jpg

The most common type depicts a weeping female figure, typically cited as a "Jewess" or more commonly Judaea personified, and a bound captive, both arranged around a palm tree. The palm tree here to signify that it is the foreign land of Judea, there being few palm trees in Rome itself. Incidentally, one of the most interesting coins I have seen was one struck by the rebels of the Second Jewish (Bar Kochba) revolt. They struck most of their coins directly over Roman coins. In this particular example, the revolt coin was struck over a JUDAEA CAPTA. I imagine that it must have made the man who struck it inordinately proud.

Augustus struck coins commemorating the annexation of Egypt.

http://www.wildwinds.com/coins/ric/augustus/RIC_0544var.jpg

The three types he struck all have a crocodile with the AEGYPT CAPTA legend on the reverse. The crocodile would have been easily recognizable at the time as a symbol of Egypt, particularly amongst a mostly illiterate populace. I don't find any coins in this series with a palm tree. Perhaps you are thinking of this one?

http://www.wildwinds.com/coins/ric/augustus/RIC_0158_SD_cm.jpg

The crocodile here is chained to a palm tree. COL NEM refers to the Roman colonia at Nemausus, where many veterans of Augustus' (then Octavian's) army were settled following their victory over the forces of Cleopatra and Marc Antony. The chained crocodile here shows their victory over the Egyptians.

Augustus also minted coins celebrating the conquest of Armenia

http://www.wildwinds.com/coins/ric/augustus/RIC_0515.jpg

Here the reverse shows a bow and quiver of arrows, and what is usually described as "an Armenian crown". I'm not sure if the average Roman would have looked at the image and called it an Armenian crown or not. Certainly the iconography is not as distinctive as a crocodile.

Trajan has a series of coins for his conquest of Dacia.

http://www.wildwinds.com/coins/ric/trajan/RIC_0096.jpg

The DAC CAP across the bottom of the coin is for Dacia Capta. The remainder of the text are Trajan's titles, of which there were many, and which he was apparently fond of having listed in energy on his coinage. The figure is Dacia personified, bound, and sitting on a pile of captured weapons.

Domitian had a number of GERMANIA CAPTA types

http://www.wildwinds.com/coins/ric/domitian/RIC_0274.jpg

The figures are usually described as a female captive in mourning, and a bound male captive, both standing under a military trophy.

Those are all the conquest commemoratives I can think of off the top of my head. In each case, the imagery on the coin is used to convey a sense of either the place that was defeated, the the fact that they were defeated, or in some cases both. The palm tree is generally associated with the Judea series. It wasn't a symbol of conquest per se, but in this case an emblem of Judea itself.

I hope this was helpful.