I'm interested in learning how to create an image gallery. But I need to know the CSS and how to create a thumbnail of the image.
I think your question is very broad. There are heaps of resources available on the web and you will find most of it with google.
If you don't have an image editor you can find free software that will resize images to make thumbnails. This is probably only one of many examples:
As for the CSS there's no substitute really, for understanding the code. You could start with w3schools.
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