I happen to be a Pennsylvania lawyer practicing intellectual property law. You should understand that nothing posted publicly here is "legal advice," and this response to your question doesn't form an attorney-client relationship.
The answer of Sarah Grosse of Florida is right as far as it goes. "Quantum meruit" is a lawyer's fancy way of saying that you are entitled to the value the work you did in reasonable expectation of compensation.
Another way to frame the issue is that it is a "contract implied in fact." That is, under the facts of the matter, you had a contract with the band, and you are entitled to be compensated for whatever a reasonable observer would infer you and the band had agreed to.
It's unlikely that their adoption of the logo as a trademark would be found to constitute "theft" of your property. You designed the logo(s) and voluntarily sent various versions to them. You expected them to adopt it as a trademark. Their adoption of the trademark would better be framed as "acceptance" of a contract with you to pay you fair value than as "theft."
If the logo contains enough "expression," it might also constitute a copyrightable graphical work. And it's possible that their failure to pay you for that copyrighted work would mean that they don't own the copyright and that their unauthorized duplication of the image would also be "copyright infringement."
For further information on intellectual property matters, visit my website at www.elman.com.
