There are other alternatives. One key feature to make it easier is an application that has "layers". Do a Google search for Mac compatible editing programs that have layers. You put the shark on one layer, and put the pool on the bottom layer (background) underneath it, merge/flatten/combine/different words for basically the same thing and walla! you are done. This assumes you have already removed the image surrounding the shark.
Pages has the ability to work with layers... so while it may have seemed facetious, it was not. The second "feature" you need is the ability to remove the parts in the photo of the shark that are not the shark itself. That's the tougher one. But... if you can 'paint' those pixels oner with white then I believe you can use an "alpha layer". Essentially, painting pixels you don't' want to see with white and cutting them out entirely creates the same look. Adobe products use the lasso tool (primarily) for selecting pixels you can then delete (cut out.) I don't know what other applications call the lasso tool. Alternatively you use the lasso tool to select the shark and copy those pixels, which are then pasted on top of the pool.
For someone who knows what they are doing this is not too big a job, depending on the photo to the shark and how complicated it is. For someone who hasn't done this before and doesn't have the tools - you are starting to discover why people get paid to do this.
If you are in a rush, pay one of your friends to do the job. This is worth at least 2 beers. If you have time and don't want to spend any money, then find an editing program similar to Photoshop Elements (you don't need the full Photoshop - elements will work fine I believe) and use the trial version. Unless you plan on using Gimp for a lot of other things - don't bother with it. The learning curve to get x11 installed, then to install Gimp, then to learn Gimp is worth at least a twofer (24 in Candianese) of beers to your friend who edits images. Trust me.... Gimp is a bear.
