total newbie: simple image manipulation

Hello there,

A friend told me about LibGD as a good library for image manipulation.
However, I have little to no experience in working with libraries such 
as this one, so I'd need some help in getting started.

I'm writing a script in AutoHotkey (a Windows scripting language)*, and 
what I need to do is
a) convert a PNG image to BMP, and
b) embed a text caption in the image (e.g. adding the source URL to the
    bottom right of the image).

While this should be a simple task (I guess), I have no idea where to 
start....
Could someone please point me in the right direction?


Thanks!

F.


* In case that helps, AHK's DllCall function is described here:
   http://www.autohotkey.com/docs/commands/DllCall.htm