You don't have to go to Jæren to find beaches, although the Jæren beaches are bigger and more sandy. The city beaches are spread along Gandsfjorden, they are small and cozy. Go by bus no 4 to Rosenli or no 6 to Godalen and walk between the buildings down walkways to the fjord (see my Rosenli tip or the "bike or hike" tip right above for more info and linked map.) The beach at Vaulen is a bit larger than the Rosenli and Godalen ones, several buses take you there: No 1, 2 and 3 for sure. The depicted beach is situated by Rosenli.
When I lived in Norway, in Sandnes, I used to LOVE going for long walks on the wide Jaeren beaches. Jaeren is the (largely agricultural) are that stretches from below Stavanger to the south. There is a lot of farmland and cows, but also incredible beaches!
I think this area is often underestimated. Many people come to Norway because of fjords and mountains. Jaeren has, not typical for Norway, wide sandy beaches. The northern north sea waves came rolling with great force and it's almost always windy. For someone like me, who lives quite far from the sea, it was paradise hopping on the train and going to the beach, even in January or February. Dress warm, bundle up your hair (if you're a girl) and try not to be blown away!
Some nice beaches are, for example, Sola or Brusand.