No problem Giovanni.
The easiest method to do what you want is what I did earlier today and have done a number of times and it works great.
Put two markers (press the m key for this) on your timeline, one at the beginning and one at the end of your desired time.
Place the series of pictures on your timeline, beginning at the first marker. It doesn’t matter how long they’re on there for as we’ll change that in a moment. Apply a dissolve or any other transition between each one if you want that.
Click the first image, hold down the Shift key and click the last image in the sequence (this selects all of them) and press the g key to group them all together.
Now place your cursor at the far right edge of the last image. Hold down the CTRL key and drag everything to the right until you get to the second marker and let go.
You should see a squiggly line (looks like a series of W joined together) that looks like a small wave across all these images. That’s an indication that things have been slowed down or sped up.
That’s all there is to it.
The same thing works if you’re trying to shrink a group of images to fit a smaller amount of time. Click the first one, shift+click the last one, group them and then shrink to fit.
Have fun!