How to add your Company Logo  in salesforce home Tab:

Login into your salesforce account,

upload the image under documents tab

• Go to yourname --> setup -->appsetup -->Customize-->homepage component 
• Under the custom component click New and then click next 
• Give the Name as Logo and select the type as image, click next
• Click the insert image button and select the image/logo file
• Then click save

Now we are successfully uploaded logo into salesforce, next step is we need to enable the custom component in the home page layout

• Go to yourname --> setup -->appsetup -->Customize-->homepage layout
• Click edit the layout
• Then select our component after that click next
• Move our custom component in the top of the left side column
• Then click the save button  

That's it, We are done! Company logo is added into your salesforce account click the Home button.