Get Android Jelly Bean 4.1.2 stock firmware for Samsung GT-I9100 Galaxy II smartphone from here or here. Get another Samsung firmware from here.
Archive name: SAMSUNG_I9100 4.1.2 fw
Image: SAMSUMG_I9100_SJDS_20150316 & SAMSUMG_I9100_SJJL_20150316
Archive file size: 619 MB
Support: Samsung Exynos 4210
Build number: GT-I9100-user 4.1.2 JZO54K I9100ZSMSA release-keys
Caution: Please do not update firmware unless you know what you are doing, you really want it or it is absolutely necessary. Firmware updates may occasionally have bad results, like bricking your device, temporarily or permanently. The firmware update process is done entirely at your own risk. Before attempting to update the firmware, back up your data.