I'm now reading some books of R, but I want to know if I can use this language as I use Perl or Ruby. Things like:
But it's usual and simple(as in Perl or Ruby) to do things like this?
PS: I liked this language very much, because of this I want to use it on my personal projects and spread it for my friends and at the internet.
The CRAN Task Views are reasonable starting points. So in order
Image processing: see Graphics and MedicalImaging
File compression: accessible from Base R, so try help(connection)
Use APIs: you will need to ask that question again, if you mean language bindings: yes, plenty, though no one single page for all
Interact with Internet: see above on help(connection), there are also packages that wrap curl, provide SOAP and of course the XML package.
Edit: And I forgot to stress that R as a statistical language and environment is more domain-specific than either Ruby or Python so the comparisons aren't entirely appropriate. But you can also code Gtk2 guis in R if you feel like it...