A) Any image should work in your signature, just how it would work in a message. If you wanted an animated image such as the following one as your avatar, however, it would need to be at most 100x100 (150x100 with Width Extension), under 254KB and you would need the Avatar Animation item from the shop.
Any image from the internet should work if you use the Insert Image option on here or just use the IMG tags yourself. If what you want is too large, I can resize it for you?
Hi Ive been cycling for 2 hours and was too tired later, fell asleep. Got another fairly popular villager in boxes and have to sell it first, sorry for the inconvenience >< trying my best to hurry.