Welcome to the forum Melani. No liners are optional. You just need to make sure that your pan is properly prepped to ensure the muffins do not stick. I recommend making baker’s grease. Simply mix equal parts BY WEIGHT of vegetable oil, shortening, and flour. You can mix as little as 10 grams each. Or if you bake frequently, You can mix a batch of 300g and stored in an airtight container in the refrigerator. It will keep up to six months.
I'm with NorCal here. Muffins are usually pretty easy to make. The sticking point is that they stick and even with a non-stick pan or silicone mold you can end up with muffin pieces. So either use the liners or the grease. You'll be glad you did. Also, give them the toothpick test about 5 minutes before they're supposed to be ready. Certain ovens or baking tins can bake your muffins faster than expected.
Yes @melani the blueberry muffins look great.
You got some very nice rise on them too. It’s hard to get a good rise on muffins Because of the mixing method, there’s no creaming of the butter and sugar.