how do i make a photo slideshow

Say if they only accept gif files...

There are lots of software to do it, easily.

You can use a gif animator software such as

http://www.coffeecup.com/gif-animator

There should be freewares as well for gif animation/slide shows.

On your PC, you can do the same with Windows movie maker, to make a slide show. But I don't think movie maker saves in gif file though.