Picasa is Google's image application. You can download the full application for Windows or Mac and do image editing to your heart's content. However, on your desktop Picasa automatically goes through your hard drive and catalogs every file with a jpg or gif extension (png may not always get included). Personally, I don't like that-but my needs are simple.
The online version of Picasa allows you to make Web albums of photos from your computer by choosing, uploading, dragging to arrange, and setting the size of the slideshow as well as a few other properties. To get into properties, create the album, then right-click it (Control-click on the Mac) and choose what you want.
To include it on a Google web page, just use the Insert menu and look for Picasa image or Picasa web slideshow. When you click on the slideshow gadget, you'll see a few obvious icons for positioning and the familiar wrench (or nut, or whatever it is). clicking on that allows to size the show and have it play automatically or not. Simple!
A Plant album (photos of different sizes):
A smaller album, also various sizes: