Resizing images contained in oid fields

Hello,

I have a table where an oid field is used for saving images. I'm thinking
about getting a little snapshot of all images without downloading the full
images. I only want to download some entire images. I'm thinking in something
like this:

create table images (
id serial primary key,
title varchar not null,
photo_id oid
);

select title, snapshot(photo_oid, 120, 120) as snap from images

In this case, the snapshot function returns the resized snapshot of the original
image.

I think that the core of function could be similar to this:
1) Obtain the image
2) Resize the Image to new size
3) Return the Resized Image

Do you know if there exists a function like this ("snapshot")?
Otherwise, What is the type returned by the function ? What suitable procedure
language should I use ? What image library for the redimension ?

Thanks you very much.