Drupal for a portfolio- Images as nodes or something else?
I'd go with the image-per-node option if you think you may at any point want to add additional information per image.  You can call the content type 'Project' or something like that and put an image field on the content type.  You can have an 'unlimited' checked as the number of images allowed in that field or only allow for a single sample image.  Up to you.

The point is - imagine that down the road you want to add extra information about that particular screenshot/project, such as the date you worked on it, or who your client was, or a link to the live site or something like that.  If you make a separate node for each project, you have the flexibility to add fields in the future.

If your entire portfolio is a single node with a bunch of images, you don't have the option to add extra information to explain those images later without re-doing the entire setup.

You can always just display them as a slideshow with no extra information, regardless of which option you choose, but I'd go with the one that allows for flexibility in the long term.  :-)