Hello! I do have a question. If I want to have a custom pet, do I need to buy a new pet, or change the one I currently have? Also, would I be able to use this particular image for one of my wolf-customs?

If you're talking about custom overlays those are just a new image used in place of the original pet image, so you can either create a new pet or use one that you already have but you do have to have a pet you want to assign the custom overlay to.
As for the image itself, probably the only way to get a concrete answer is to submit it and see if it gets approved. We can't really give you a concrete yes or no since this is sort of a rules question, but I wouldn't recommend filing a ticket in this case since generally UAs can't "pre-approve" images by telling you yes or no. But you get a refund of the CSC if it's not approved (and you can always use it as a non-custom overlay just with coding on your pet's profile). The full rules are on the page I linked above, but the most glaring one is your image has to have a white or transparent background on it
thank you!
and how to do 'non-custom overlay just with coding on your pet's profile'? it doesn't cost any C$C?
and thanks for all the help, im newbie ^^
If you go to your Pets page and click the little dropdown arrow in the top right next to the pet whose profile you want to edit, click Edit Description and you can add coding to their profile and you could just use coding to put in the image you want to use without it being an "official" custom overlay. It will only show up on their profile and not elsewhere on the site, but I believe the rules are a little bit looser (e.g. you don't need to stick to a particular size of canvas or background color but it would still need to be your own art or art you have permission to use, etc.)
I don't know what the code is for that off hand, but you could ask in the Profiles and graphics discussion forums
go to the page where you edit your pet's description, as explained, and paste this code in the text box:
<style>
{
width: ___px; height: ___px;
background: url(IMAGE URL) no-repeat;
background-size: ___px;
}
</style>
replace the blank spaces with the dimensions you want your image to be displayed as, and IMAGE URL with the direct URL of the image you'll be using as the overlay.