Right, time to get technical (well, for me anyway...). OK. For the clothing system, I am going to use examples (I know you would probably understand but I need to use them to get my thoughts right). Basically, my idea (and probably other users as well) is that you have a layer system. Each item is assigned a different numbered level. For example let's say that we have 5 items/clothes/whatever (each with a different number):
A hat
A hairstyle
A top
A skirt (the example is gonna be a girl)
And some trousers/leggings (they go under skirts sometimes)
Here is my base image (note that I did this when extremely tired, not being interested in drawing, not having my tablet, and therefore using my mouse pad... sorry for the rubbishness and the stick figuriness XD):
Here is my image before any layers or clothes or whatever:
Now, lets say that you added a hat (ignore what level it would be for now).... the image would now look like this:
Now you can add levels.... the actual adoptable would be level '1' (AKA the bottom layer) and the hat, which is above the adoptable would become level '2'. Next image, time to add hair!
The hair would be above the adoptable layer (level 1) (it could be below but for this it's above). But, it's below the hat. Therefore the hat can no longer be a level 2 item and must be a level 3 item. (This would be decided before hand and you obviously have to be careful when you decide what level each thing would be, in case you mess it up). The levels are:
The adoptable is level '1'
The hair is level '2'
And the hat is level '3'
Next one (adding a top!):
With the top, you can see that it is above the adoptable image, above the hair image (this is so that the hair doesn't appear as if it is weirdly disproportioned) and doesn't really affect the hat image. I do understand that there are some flaws with this idea because you can have hair that goes over tops (maybe a few strands) but this is just a really, REALLY basic idea... as in not really thought through 101% XD
The levels are now:
The adoptable, still level 1
The hair is still level 2 (above the adoptable)
The top is gonna be level 3 (above both)
And the hat will just be bumped to level 4 (even though it isn't affected by the top)
Now lets do the next, the skirt!:
For this the skirt is above the adoptable (level 1), and the top (level 3). It doesn't affect the hat or hair. The levels are now:
Adoptable, level 1
Hair, level 2
Top, level 3
Skirt, level 4 (goes above all of the images except the hat, but obviously you could have it go below the top if you want...)
Hat, level 5 (is bumped up again)
The hat will just get bumped up again and again (but can be decided by the admin person/owner person)
Phew, now lets do the final image which is the leggings/trousers:
Really, the trousers go below every layer EXCEPT the adoptable image, which will always (in this example) be level 1. You could have backgrounds though that are level 1 and the adoptable image above it...
New levels:
Adoptable, level 1
Trousers, level 2 (pretty much because it is below everything)
Hair, level 3 (bumped up)
Top, level 4 (bumped up)
Skirt, level 5 (bumped up)
And hat, level 6 (bumped up again)
That is pretty much it, everything has a layer predetermined by the owner of the site (it will be a LOT of work, but worth it when finished). You could maybe even name each level? Such as 'hats', 'tops', 'trousers', or 'backgrounds'. That would just be so you can see what category everything fits into. It'll be easier and more organised if you name them (maybe add it into the admin CP, so that you can add levels, rename them and change their orders and stuff). This is a REALLY basic idea, and you would maybe have to only do it with adoptables that use the same outline.
Another idea would allow you to actually add different images for EACH individual item, therefore allowing one item, with one image from the shop, automatically detect what adoptable it is (or species, maybe upload different images for each species, and it detects what species the creature is and so when you buy it, it fits every species, or the ones that the owner has said it would fit). This would allow you to conserve space in the shops so that you don't have ten different versions of the same item in the same colour clogging up the whole store page... just an idea.
It would also allow you to have items that can only be equipped to a certain species, like a bridle on a horse, seeing as the image would only fit the horses shape, and not for example a dog's. Alright, I think this is enough for the clothing idea (you could have a page dedicated to your clothes in your inventory, maybe called a wardrobe? Or whatever you want). Let's move on.