Hi Elizabeth,
FIRST has some logos and images online that you can use. The Resource Library is a really great tool you can use. Here is the link:
On the left hand side, you can filter for content type (select FLL) and tags (you could browse through things like logos, fundraising, etc.). You can find a lot of good resources there.
Here is one I found specific to the Animal Allies season by doing what I outlined above:
You can also download the FIRST logos here:
You could use those images above to make a nice thank you certificate specific to your team. Hope that helps!
Sonya