Am I able to upgrade the Processor from Core i5 to Core i7?

This is tricky! Yes you can replace your CPU with an i7 chip Remplacement du CPU de l'iMac Intel 21,5" Retina 4K (2017)

But! I don’t think that’s what I would start with. You have the right idea of installing more RAM while you can go to 32 GB you may find you only need 16 GB and putting more effort into your drive storage as most photo editing apps also use a lot of drive as well as RAM iMac Intel 21.5" Retina 4K Display (2017) Blade SSD Replacement and Remplacement de la RAM dans un iMac Intel 21.5" Retina 4K 2017

So what to do? If you have a Fusion Drive’d system then you can replace the PCIe blade drive to a larger one like a 1TB OWC Aura Pro X2 SSD. If you don’t have a Fusion Drive’d system or bought a SSD model either you won’t have the PCIe slot to put the drive in so you’ll need to look at getting a Thunderbolt external drive like OWC Envoy Pro EX Making it your boot drive! As its faster than the SATA III (6.0 Gb/s) HDD your system has.

So what we are tring to do is leverage the fastest drive I/O you system can provide. We what to set it up as the boot drive as well as where your apps are located. Your system will leverage this drive for virtual RAM and caching. And by default your photo editing app will use it as its scratch space so you don’t want to load it up! I have a 2 TB SSD in my Mac Pro and I use if for ultra large photo editing and its choking as I’m pushing this systems limits often. I leave 2/3rds of the drive empty! I’m at the extreme side of photo editing, so I’m sure you’ll be fine with a 1 TB drive.

So what about the SATA port? I would swap out the slow 5200 RPM drive Apple put in and put in a good 2.5” SSD like a Samsung 860 EVO Plus. It will hold your library and work sets Changement disque dur iMac Intel 21,5" Retina 4K (2017)

Hopefully that gets you where you want to get too. Its a big investment and I do strongly recommend you don’t try doing too much at once! I would focus on the RAM and blade SSD (or external SSD) as the first actions, then upgrade the HDD to a SSD, lastly the CPU.