Retailer logos are often used to display links to your retail partners. According to the documentation, the logo image is a "URL of the retailer logo. Should be at least 500px wide and a 24-bit transparent background PNG."
To upload a new logo, click Retailers, then the name of the retailer. Then select "Upload File" as shown below and choose an appropriate file from your computer:
Obtaining an updated logo can be done in a few ways:
Perform an image search via Google for the retailer name and include " transparent logo png". For example, "best buy logo transparent png". Retailers change logos often so be sure to reference the retailer Website to ensure the current logo is selected.
View the affiliate program profile. Depending on the affiliate network in question, the retailer logo is often available for download. Each affiliate network has a section for the advertise programs joined. The program profile might list an icon or banners for download. Be sure the transparency, dimensions and file format are appropriate for use.
Obtain the logo directly from the retailer Website. Retailers often list approved logos and design standards on their Website.