Create a content type called "portfolio item".
Add 2 fields, one for the image (file), and one called link (text). This will contain the link to the external site.
Now you can create a view listing all of your portfolio items. Add the link field but exclude it from the view. Then you can use that to turn the image into a link.
There are a variety of modules that enable you to re-order nodes. There is an overview of them all here but Nodequeue is the most popular and looks like it would be suitable for your needs.