There could be varying reasons; hormonal, old blood that has yet to be cleared from your body and also cervical polyps. Gynaes would usually not be able to pinpoint the exact cause of bleeding but prescribe progesterone pills to support your pregnancy.
I had spotting during my first trimester to early second trimester due to cervical polyps and was prescribed with duphaston and utrogeston which are progesterone pills even though it was discovered I had a cervical polyp. Later on at week 17, my gynae removed the polyp and I never had spotting/bleeding ever since then. Am in week 29 now.
In general, old blood (brown) in little amounts is considered ok and not a cause for alarm. But if you’re afraid, do get it checked out by your gynae. Based on the photo, it looks quite normal to me.
Jo