ArrayList. it was designed from the start to be resizeable.

also, you can give it an initial size, even if it's a guess, and that'll help. ditto ensureCapacity()

see the java reference
http://download.oracle.com/javase/1.4.2/docs/api/java/util/ArrayList.html

(arraylist, as the name suggests, uses an array to implement the list. but it's more intelligent about resizing than append() which (i'm guessing) allocates space for the new array and copies everything over (every single time))

I'd say an ArrayList if you don't know the size.  I believe an append essentially does an array copy each time, where the arraylist does an array copy only periodically when it exceeds it's allocated reserve.

I understand, but is the allocate reserve setted with ensureCapacity or it has its own space by default? (i guess beacuse of the trimToSize method...)

The ensureCapacity just sets the initial memory, so that it doesn't have to do an internal array copy to grow until after it reaches the ensureCapacity limit.  So it can go above this, it's just the initial memory allocation.  It improves load time if you have a good idea of the initial size.

ok, thank you, this helps to choose ArrayList as a better way to mess with changing arrays.

Moreover, nor the initial memory - new ArrayList(5) - neither - ensureCapacity(5) - return ArrayList.size() as 5, but 0 since it's filled with something, this is also interesting...

If you don't know the number of images you are pratically obliged to use ArrayList since it gives you the possibility to add objects without the problem of a fixed upper bound(that is a problem with arrays).

So the matter is the position of the element in memory ?

And what if I create 2 ArrayList and I add images randomly in these ArrayList ? Do they share adjacent space in memory ? (i guess however that if ensureCapacity() is used they have at least some reserved space...)

And if during the runtime I declare one of this ArrayList whit a new empty one, what happen to the used memory? Is it refreshed?

From the classic processing blue book:
" Tips for working with arrays
Adding one value at a time to an array is slower than doubling the size of the array when it’s full. If a program will be continually adding data to the end of an array, use the expand() function once each time the array fills up in place of running append() many times. Code 33-19 (p. 309) shows how to manage a growing array with expand().  The arraycopy() function is the fastest way of copying data from one array to another. Copying the data from one array to another inside a for structure is much slower when copying data from large arrays. The arraycopy() function is demonstrated in code 33-20 (p. 310). Arrays are also much faster (sometimes 2*) than the Java classes ArrayList and Vector."
Hope that helps...
Pd.: this is also interesting: http://www.roseindia.net/javatutorials/linkedlistvsarraylist.shtml

Old discussion...
Array lists have some drawbacks: they can be a bit slower, indeed (Vector more than ArrayList, because they are synchronized (related to concurrent access, in threads), that's why they are quite deprecated.
As said, in most sketches, the difference of speed, at access time, between array lists and plain arrays is probably unnoticeable, the main bottleneck being the graphical operations!

If that's really critical, ie. if, by using a profiler, you find out you really loose a significant amount of time in the array list access, and if you have two separate phases (first, building the list, second, using it), you can copy the content of the array list into an array.

Ah, there is a case where the difference of speed can be important: when you use an array list to store primitives, like simple ints or floats. To access them, not only you have the little get() function call overhead, but also the unboxing overhead, since primitives must be wrapped in their equivalent object (Integer vs. int, Float vs. float, etc.). This can be costly, and also cause a large memory overhead too!

Array lists are better used with objects, from PVector to your sophisticated own classes. Here, they generally behave as well as arrays for access purpose and much better for extensible storage.

Note: I wrote a note about this in the Technical FAQ ( Why use ArrayList instead of array with append()? but Zoho tends to break these links).