Mark asked: "I've got 4 images with the same keyword but when I do a search on my website, only one appears. Why?"
Mark, that's because you have enabled the "group search results by gallery" in your website General Settings.
Compare the following screenshots, searching for "Road signs" on a website:
On this example website, there are 7 images matching the "Road sign" search, all in the same gallery. So when grouping results by gallery, only one thumbnail appears, for that gallery (named "Road Signs").
What is grouping search results good for? If you have many galleries with very similar content within the same gallery, it makes the search experience more efficient and pleasant for your visitors, as similar images will not clog the result pages.
On most websites, the results are ungrouped by default. This default behavior can be changed in the website General Settings.