Squiby offers a service for making your own growable adoptables, but they have a size limit (it's very small)
If you want to do growables here have you considered using the oekaki?
You could draw all the stages linearts in Adoptable Oekaki and then colour a pet, and if you want it to grow, edit the image and change it to the next stage up. It would require you to colour each stage but it would end up being the same image
Other than that, as you say, photobucket is a very easy way to make growing adopts since they use the same url.