The following images shows the tyre sizes of my hybrid bike. I need to buy an inner tube with presta valve. I can't seem to figure out which size I should buy though. Can someone please point me to the right tube size for my bike?
Your tire size is 700x38, meaning the overall width of the tire is 38 mm. Inner tubes are sold in ranges of sizes rather than for specific widths.
eg 700x23-25 - means this is suitable for 23 to 25 mm; 700x28-32 - suitable for 28 to 32 mm; 700x35-42 - suitable for 35 to 42 mm; and so on. So you need to find a tube has your tire width (38 mm) that falls within the range that's on the packaging of the tube you're looking at. And make sure that it is with a Presta valve rather than Schrader.
Edit: just to be clear, the above ranges are just examples. Different manufacturers may have slightly different ranges.