How to retrieve a value from wp database?

Please, I would like to know how I should be able to find a certain value from the wordpress database, in order to use it.

For example, this generic situation:

I have to manage a virtual cart, and i need to add a small thumbnail image of each product next to the item name.

In the <IMG> tag, the SRC url of a thumbnail image is stored in a sub-table of wordpress database (generated by a plugin).

Whatching the database with phpmyadmin I know the name of the field, but how can I recall this url from the database?