There are two images required for your Bucky Box account, which you can upload in your Settings page.
The first is for the banner at the top of the webstore, which should be in the scale 980px x 160px.
The second is for the picture on the left of the webstore, which should be in the scale 600px x 400px.
Both will automatically crop and scale to an appropriate size if you upload something outside of these dimensions, with the mid-point of the image as the anchor. If you want to get an ideal scaling, please make sure the images are in an appropriate scale ratio or are the exact sizes above.
Here is how the images are used in the webstore:https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-ash4/374988_447869445296301_2052122095_n.jpg
If you don't have photo editing software, you can use online services such as Pixlr, where you can edit Image Size.