Well, I haven't used PowerPoint since school last year and I've never made a banner in it before. But you can download GIMP for free by searching it on google and I can help you from there for free. =)
1) I only know the attributes for siggy banners, so you can open up a canvas at 500 x 250.
2) Open up another canvas without closing the other one and make sure it's 460 x 170
3) Copy and paste the picture you want the background of the banner to be, or if you want a solid color, just fill the whole thing up with the desired color.
4) Outline the entire box with a thin line of black by clicking on one corner and holding the shift button to create a straight line.
5) Cut and paste the whole thing into the first empty canvas.
6) The second canvas should be empty since you pasted it into the first
7) Copy and paste the picture you want on the banner into the second empty canvas
8) Using the selecting wand (it looks like a tiny red stick), click the area surrounding the picture in the second canvas
9) Click select, invert
10) Copy it and paste it onto the first canvas with the banner background
11) With the selecting wand, you can move it around. Click away from it when you want to drop it there.
12) If you want to add more pictures, select the picture on the second cavas and delete it then use it to copy and paste more pictures.
13) Make sure you have room to put text
14) Click the text icon in the tool bar and put what you want in what font and color in the space
15) Save it with the name and at the end, put .jpg
16) It will ask you to export, so say yes and put the quality up to 100 when that pops up
17) Once it's saved, go to www.tinypic.com and upload it from your computer
18) When it comes up, click on the picture and click on the link that says something about raw image
19) It will bring you to the link. You can use it for HTML brackets. The URL is the URL to your banner. :)
Hope I helped!