If you have photoshop available to you then that is the most powerful option for cropping and resizing images, but it's also expensive. If you need a free option (and need a tutorial on how to use it) then read on.
Step 1: Figure out what size image you need. If you want to swap out an image (or add a new banner to your site) then you should make sure the sizing is consistent with what is already on your site. To check what size in image is, right click on the image you want to replace and select "open image in a new tab". Then mouse over the new tab and it should show you the filename + dimensions, as seen here:
As you can see, the dimenions of that image area are 1673x456.
Step 2: Now that we know the size of the image area, we need to crop a new image to those same dimensions! Assuming you already have an image ready (maybe from one of your brands), go to http://imageresize.org/. Click "Choose file" and select your file, then click "Upload":
Step 3, you need to crop it! Click on "Crop", and then in "Aspect Ratio" select "Custom" and type in the numbers that we found in Step 1. In this case it's 1673x456.
Once you've typed that it, you'll see a box that you can enlarge or shrink down and move. Put that box where you want and click process image. Then click the small blue Download button and you're done! Note, do not press the large green buttons on the site. Those are ads. See below:
You now have an image in the proper dimensions for your site that you can upload.
Let us know if you need help or have questions.