Ok I'm relatively new to PHP but I have enough experience to help a bit. But first a few questions. First what are you trying to do? And have you experimented with PHP at all? I'm going to assume you're trying to disply 100 images on your page, considering you said you need to load 100 photos. Ok so..
<head> <title>Prueba 1</title> </head>
<body> <p>Este es el sitio de imagenes de prueba</p>
$Img = 'hola/Foto1.jpg';
$Img = 'hola/Foto2.jpg';
$Img = 'hola/Foto3.jpg';
$Img = 'hola/Foto4.jpg';
$Img = 'hola/Foto5.jpg';
<img src= "<?$Img;?>" width ="80" height = "50" alt = "Primera foto">
You mentioned you think a for loop would work. Yes it would so use one... just making a variable equal to a value several times defeats the purpose.
Also as said above, you need to declare, or let the server know, $img is a variable array.
and you can do that by stating $img = array(), however if you want things to be in the array you either have to add it in the declared statement or use the array_push function.
In PHP, 'echo' will display whatever is after it. 'echo $img;' will just display whatever was put in that variable, in this case will display 'hola/Foto5'.
There are a lot of issue with your code and what I've mentioned was just the start. The project you're on maybe too much for you right now. Practice first making a variable array work without making them images. Make them just plain text and try to display several different words. And then learn... well what a for loop is because you're not using one.