So, I know "what" it is, but that's pretty much it, I'm almost sure I did find a topic about it before explaining what you could do or not but I can't find it anymore x_x'
So, my questions...
1 - Can you use a custom image for your pet in your pet profile without actually paying for this custom overlay feature? Like, if I draw Ankh dancing can I use it with custom code in his profile, or at least add it in the page without changing the original picture of him? If the second option is allowed, is there a limit for how many different images of the pet I can add in his profile?
2 - What exactly is allowed for the payed version? How much can you change your pet? Can you add clothing, change the pet colors, add hairfurspikesetc to their body? As an example, if I had a Reborn pet, could I draw it blue instead of red? Or maybe get rid of the floating balls of the Darkmatter pets? What about a Common Anyu with wings or a furryterrestrial version of a Torrent? How much can you "humanify" them?
3 - If the free custom images are allowed, the exact same rules apply?
4 - Is there any way to check if your image is okay before actually adding it to your pet's profile or trying to get the payed custom overlay?
And this last question is not really related to anything, but where exactly can we post fanarts, not only from our pets but also from NPC's? I'm aware we have the Creative Corner but I usually see people asking about tablets or asking for hints; is it the right place to just post your fanarts without actually trying to "start a discussion"?
Sorry for the broken english and so many questions 
You can use an image without paying for it, even to replace the default image as long as you don't hide the text that says what color/species/gender your pet is
This page should clear up questions about waht is allowed. No anthros at all are allowed, but you can do a lot of other things (I've seen several PS winners whose custom pet images are just a regular dog vs. a Subeta pet species). Images on profiles don't need to follow these rules, but they might disqualify you from winning Spotlight (I'm not 100% sure on what is/isn't allowed for Spotlight off hand)
When you upload a custom image, it has to be approved by a staff member before you can use it. That's how you find out if it's okay. Your CSC gets refunded if it's not.
Also you're probably looking for the Art Discussion forum
yes, you can code in an overlay without using the paid custom overlay feature. there is no limit to how many images of the pet you can have on the page.
you can edit existing pet art to add or remove features, change colors, etc. you can also draw (or commission someone else to draw) a completely new image. the pet can be anthro (a humanoid animal) but cannot be a straight-up human. the main restriction is that your overlay cannot misrepresent the pet as being an existing color / species that is different than what it actually is. a more detailed list of the rules can be found here.
the only rule for free overlays that you code directly into your pet's profile is that you cannot misrepresent the pet's color or species. other than that, it can be whatever you want, you don't need any approval or anything like that.
you could ask for feedback from other users on the overlay, but if you want a definite official answer you might want to file a ticket, though i'm not sure a staff member will review the image unless you submit it. if you submit the overlay and it gets rejected you will be informed of the reason for rejection and your CSC will be refunded.
[edit] ah, ol' squiddy beat me to it again.