One good and rather cheap program I'd really recommend is ACDSEE. It provides a slide show option and you can modify pictures in many ways (including size and compression). Often you can get it from one of the large online stores for real cheap. I bought the last year's model and it was about 1/3 of the price as the current version, under $20 if I remember correctly.
It's not as inclusive as Photoshop, which I'd also recommend, but it's FAR cheaper.