I bought one of these just earlier this year after it got very good reviews over on MTBR. Raveman CR-500. Designed with a cut-off beam pattern in mind so as not to blind people using typical paved trails. Charges with typical USB phone charger. Comes with a quick-release mount so easy to take with you. Includes a removable wired remote so if you wish you can change modes from the remote button. When off the mount it easily slips into a pocket.

Personally I've been a dedicated night rider for about 20 some years. I'm used to lamps that output more light but this little light gets the job done. I'm not sure it's an actual 500 lumen on high ( maybe 400 ) but so far I've not had any major problems while using it. $49.99...includes shipping I believe. Vendor service has been well reviewed.

Oh, almost forgot...Looking at the photo of your bike I'm guessing you will need a rear light as well. Doesn't look like much room on your seat post so maybe something to attach to a seat stay or clip onto your clothing or bag. If you need recommendations just ask.