Yes there is a simple solution. I recently helped a client set up a Customer Portal and they also wanted the Customer logo on the Portal dashboard.
Set up a table of users. You will probably find that you need this anyways to control access to the data so that each user only sees data relevant to their own company.
The key field of the table would typically be the user field. Then have a Relationship where 1 Company has Many users (that is assuming that each user will have their own userid as opposed to one per company). On the company record has a file to upload their image.
Lookup that image down to the Users table. There is a formula for doing that if you need help with that part.
But the magic is simply to show a report on the dashboard where the user user sees only their own record. (Filter userid = current user) That will be a 1 line report. Put the company logo as the only field on that report.
