In matters of Theoretical Law yes you could pursue legal action against the company!

But the way i see it these are your options:

First off, Learn from this...never give pictures to ANYONE if you care about them enough...then:

1- Talk to a lawyer and pay them for their time, and based on their advice make a decision (which will end up costing you more than is worth)

2- Talk to the company, saying how you realized they liked your photo, and would like to strike a deal for future work together....

In the end think of it as an investment...you did not get paid or get credit for your photo, but you could benefit form the whole situation in getting a contract with the company for future ads, which will give you income and credit!

Note: Regarding the issue at hand though, the matter is simple as any lawyer worth his salt would argue: if there is no Written Contract or even Verbal contract, then you simply gave away the image....its a lost cause....although technically the image is yours....

Marc