Need help with PHP code for accessing photo information

I've got an existing website written in PHP that uses some images. Those images are currently managed in a G1 installation. I access them by going directly to the local album through the filesystem (e.g. <img src="albums/foo/aaa.jpg"). The pictures to be displayed are often controlled via a list of image filenames (e.g. album1/aaa.jpg).

Since G2 stores the thumbnails separately from the original images, and because I now want to be able to access the comments stored in the gallery, I need to find out how to programmatically do the following:

• Given an identifying datum about an image (preferably the album/filename, but an ID of some sort would work), obtain size and comment information about it, the thumbnail, and any intermediary scaled images.
• Given an indentifying datum about an image, retrieve that image for display.

Can somebody provide me with a pointer to any sample code that does this? I've looked in the autogenerated API docs, but I'm floundering around...there's very little context there.

Thanks!