My back pressure valve has stopped working and now coffee grounds spray out of the group head when I release the portafilter. I don't really know what the problem could be... I have attached some pictures of my machine. Can anyone help me and what steps I should take with fixing this? Is it a cleaning issue? Is it a part replacement issue?

You may want to let the folks in the know hear what specific equipment you are using...
The valve might just be clogged, did you clean and backflush with something like Cafiza regularly/recently?
Imo just waiting a short while should let the pressure fade away through the puck unless you grind so fine you end up with a choked Filter basket (my machine does not have such a valve at all, the instruction booklet says to wait until the PF stops dripping..and that works)

It DOES have a pressure release valve that used to work so I don't know why it's no longer functioning. I have not cleaned it with any sort of cleaner or chemical (lol, probably why it's not working). Any suggestions?

Btw, my machine is an espressimo 2100 by grindmaster

I don't specifics to this machine, but I'd recommend the following. Try to identify the 3 way solenoid valve. With the cover off, cycle the machine and see if you can hear or feel with your finger on the solenoid valve click. If it doesn't, then it a solenoid valve problem or a power issue to it. If it is clicking, then something is clogged somewhere. If no backflush or de scale has EVER been done on the machine, that would worry me.

Sounds like 3-way valve isn't working properly. I'd call a local espresso machine service co. for replacement of part or contact:
Grindmaster-Cecilware Corp.
4003 Collins Lane
Louisville, KY 40245 USA
Phone:
Ph: +1.502.425.4776
Toll Free: 1.800.695.4500 (US & Canada only)

Most likely is the solenoid valve that releases pressure at the end of the shot as folks have mentioned. Without knowing specifics about your machine as is fairly obscure for our area, you should be able to trace the water path to find a valve with a (typically) black box covering the valve shaft. Unplug the machine and disconnect the wires attached to the valve, then use a multi-meter to test resistance (test the terminals that had wires attached, some have three terminals, test resistance between the opposing terminals). Most solenoids I know should read between .7k and .8k ohms. If it reads ok, disassemble valve and clean. That might fix the problem. If the test does not fall in those values, replace the valve. I would contact the manufacturer for a scheduled maintenance guide and follow it. It is much cheaper to pay for routine maintenance then buying a new machine.