What do lettuce seeds look like? To answer that we need to ask another question: What is a lettuce? It's like asking "What Are Birds"? What Are Birds? We just don't know.
Moving back to our previous question of what a lettuce is, I looked in a big textbook and found out a lot of stuff. The scientific name for the common head lettuce is Lactuca sativa. Both the English name "lettuce" and the Latin name of the genus "lactuca" are derived from "lactis", the Latin word for "milk", referring to its milky juice of the plant - so no help there.
Lactuca are in the Asteraceae family, so I had a look to find out what other species were Lactuca family and found the Helianthus and the Taraxacum, which are the Sunflower and the Dandelion respectively. A word of note here, a Dandelion is not another name for a big camp cat.
In general the Asteraceae produce flowers and then seeds on little parachutes, this is called bolting. In other words, when it is time for the seeds of these plants to get the heck away they do the bolt. Asteraceae are the SAS of the flower world, and when you blow on a flower clock, you are in reality commissioning several seedy paratroopers with their RNA cargo to disperse and propogate.
The Lettuce plant has a short stem initially (a rosette growth habit), but when it blooms the stem lengthens and branches, and it produces many flower heads that look like those of dandelions, but smaller. When grown to eat, lettuce is harvested before it bolts.
Lettuce seeds not surprisingly look like Dandelion seeds but for the question "What Are Birds"? We just don't know.