The store does not resize your images once they are uploaded. All images are shown in their original size so if you are seeing a larger image than you thought it is because that is the actual size of the image. If you would like for your images to be smaller you should resize them before uploading.
There are several good free resizing programs. We recommend using Irfanview.
The store can also automatically create thumbnails of your images for you if needed. To automatically create thumbnails go to Design --> File Manager--> Create thumbnails.
Navigate to the folder you want to make thumbnails for. Then at the bottom of the page choose the size for the thumnbails and choose the button labelled Create Thumbnails.