Blow-Up is great for your portfolios! You can also use this theme in a website made with another theme, just to show your images fullscreen. The Blow-Up theme resizes images to fill browser while maintaining image dimension ratio. Adding images is really very very simple with the free Slider stacks or snippets. There's an example snippet with links to three tulip images in the downloadfile, so you can start immediately.
There are 6 themes in the download file with a vertical or a horizontal menu and both with a YouTube version and (NEW!) a Flickr version that pulls all images from Flickr.
UPDATE: added the option for a different Title font. HTML5 and new Manual. Solved issue with Slider stacks.