Use case: use image to easily show how importance a customer for company based on closed won opportunity. We can easily create a formula field in Account with Return Type = Text, example:
** Closed_Won_Amount__c is a roll up summary from Opportunity Amount.
If you notice image in the formula field above is not pointing to image hosted in some website, but it is within Salesforce itself. This is because Salesforce provide icons available for us to use.
If you need more icons and not available in above list, you can install Graphics Pack app from AppExchange, it is free from Salesforce Labs. Once installed, you will get hundreds of icons available to use, but the image path would be different.
You can open list of image from Graphics Pack Overview tab, select a category, hover the image and right click "Open image in new tab" from your web browser to get image path, example: /resource/1400195872000/GraphicsPack/16/famfamfam/silk/accept.png
Other resources with logo or icons provided by Salesforce: