Hi. When submitting a custom img for a pet, does it have to be transparent?
Yes.
Art Guidelines:
Format: 200x200 png file with either transparent or white background. Digital artwork only. Use the 'save for web' feature, avoid dithering/stray pixels etc. Image must be drawn to fit the canvas, no squashing/stretching etc. Integrate props in with the existing art quality - don't just paste objects on. Don't simply adjust the hue/saturation/colour of official artwork - this distorts quality/lines.
Here are all of the guidelines- https://subeta.net/pets_custom.php#/upload

Wow, thanks much :)
you're welcome! I just added my first custom overlay for my pet Casey so I was looking at all that info recently myself. :)

According to the very guidelines that quoted, it does not have to be transparent.
In fact, most of my custom overlays (if not all) have had white backgrounds.
Aww...what a sweet puppy ❤
How do I give credits to the person who made it for me? Just put it in pet description?
Oh, ok, cool, thanks :)
Haha you're right. ;) Yes, it can be white too. Sorry I was trying to answer quickly! :)
thank you so much! When you submit it, there is a spot that says Image Source and you should credit the person who made it there. A link to the user's profile or to a deviant art page, etc. And then it is always nice to always credit the person on your pet's bio.

Oh, dang, now I did it...I haven't put the link to profile...but I put the credits on pet's profile. It won't be accepted like this probably...oh, well...BTW, how long does it usually take to be approved?
The approval rate all depends on when they are doing it. I think they kinda do it in batches every few days or so. The fastest I ever had it approved was same day, and the longest was about 9 days (although I think that was a bit unusual).
If you get rejected, they will usually give you the ability to submit it again without having to pay again.