Since you are working in inches you will have to have an idea of how many photos and what size they are going to be in order to arrange the collage.
You can find out any photos size by using the Resize menu selection.
Go to Images, select Resize

This will tell you the photos size. If you are designing for the web you can always change the Units from inches to pixels. I also use the Zoom feature (the % amount) to reduce the collage so that I can see the whole page. I usually work at 50%.

Now open the first photo you will put into your college.
Once you have chosen you first photo and resized now go to Edit, select Copy  (or Control C). Minimize or close the photo. You should see your Blank Page. Go to Edit and select Paste  (or Control V). This will place the photo in the upper left hand corner. Move your mouse into the center of the photo, click and hold and drag the photo anywhere on the page. Click on the white background to "set your photo" - this removes the crawling dashed outline.

Continue this process with the rest of your photos.

Your collage will start to take shape. As you work remember to use the Undo button  (or Control Z) if you don't like where you pasted something. 
Also if that doesn't work you can use the Select tool  to highlight an area and then use the Scissors  the cut function (or Control X) to remove it.

When you are finished save your collage as a Jpeg. 

You can also make the white edges transparent.

Select the Transparent Color Tool 
Click on the white background

Click OK

The new image must be saved as a GIF. This file type supports transparency.
Go to File, select Save As ...

Change the file name and the file type to Graphics Interchange Format.
Click the Save button.