Photo gallery - next/prev buttons

Hi all,

I'm new to this HTML thing. So hopefully someone can help me. I got a photo gallery site with the attached coding. Now I want to be able to move to next and previous image by pressing on the buttons. I found some coding on internet for this. But now the button '<' and '>' are outside the image I would like to have the button in the image. And I dont like the size of the buttons. How can I make it smaller ? Another challenge is that the image can be horizontal or vertical so the position of the buttons should be flexiible.

Also I would like to have my thumbnails to move lower alligned with the top of my first image.

And last but not a high priority is that I would like the thumbnail which is enlarged to be highlighted.

It would be great if someone can help me with this.

Many thanks in advance
Hazy