I have messed with this a bit more now, and it is actually quite good at finding pages within my site.
i.e. if you do a search it will return pages that have images with relevant ALT text (and my alt text is for each image is generally quite descriptive).
In view of this - I have put another search box at the top of all my pages (The box at the bottom was getting very few searches unsurprisingly - and I guess there is no point in having a search box if people can't find it...)
My only reservation for now is that Google can be quite slow at indexing new pages (It is currently taking 6 days or more between crawls)..
NB - I am talking about my
Ireland Photos / Desktop Backgrounds / Irish Wallpaper web site if anyone wants to have a look..