BCD = bolt circle diameter. What that means is it's the diameter of the circle of bolts on the chainring. For your crank, the outer ring has a BCD of 104 as your crank is likely a "trekking" crank based on mountain bikes. The inner ring has a 64mm BCD. The BCD limits how small you can go on the chainring. For a 64mm BCD, you can use a 22 tooth inner ring. Just to be sure, measure between adjacent bolt holes. If the measurement is 45.3mm, it's has a 64mm BCD

One of the features of the gear calculator that djb linked to is that you can compare different gear combinations. The graphical way that the calculator is set up allows you quickly see how the gears change and interplay. For example, here's what changing your inner ring to a 22 from the 26 tooth would do to your gearing. Play around with different gear combinations. You can drag the chainwheel or cassette gears any place you want so you can see what effect changing the combinations has on the gears.

Just a note on how the calculator works. For miles per hour, the calculator gives you the result in "gear inches" which is a throwback to the old days of ordinary or "big wheel" bikes. A "gear inch" is the wheel diameter of those kinds of bikes. For kilometers per hour, the calculator uses "meters development" which is how far the bike travels per revolution of the crank. Both methods are useful and which you use usually depends on what you learn first. Personally, I can't make much sense out of meters development without thinking about it. Gear inches are easier for me to visualize. Your mileage may vary.