is a powerful package that works from the command line. Being a collection of unix command-line tools, it can be scripted at will.

It is available to the mac (fink/darwinports/macports).

One typically needs to combine a number of the different tools to achieve a result. E.g.

$ jpegtopnm input.jpg ¦ pnmscale -width 640 ¦ pnmtopng > ouput.png

converts a jpeg to a png that's 640 pixel wide.

gimp (free)

If you're more visually inclined gimp might be what you look for. It too has a unix heritage. It however uses the X11 windowing system (optionally installed on your mac alogn with OS X).

[gimp.org...]

Gimp allows scripting (script-fu)

aperture (for pay)

[I don't own a copy]

From apple, tremendously powerful, and relatively quite affordable (esp. when comparing to e.g. PhotoShop.)

Automator can be used to script it AFAIK,

There is a free 30 day trial available from Apple.
[apple.com...]

iphoto (for pay, but included with every new mac)

iphoto is part of iLife, I guess we all have some version that came with our mac (if it's bought in the last few years anyway). Even if you upgraded the OS you can most likely find it on the 2nd re-install dvd.

Compared to the above this is likely the easiest to learn but unfortunately also the easiest to not be up to the task.

I was able to find a free program ( ShapeCollage) that does pretty much exactly what I was looking for .