Here are some useful websites:

https://saucenao.com/ 

Mostly used image search engine, mainly focus on pixiv and exhentai, if you want to find imgs on boorus, I suggest you use this:

https://iqdb.org/ 

I use them to search image on:

gelbooru(https://gelbooru.com/)
danbooru(https://danbooru.donmai.us/)
yandere(https://yande.re/)

and other sites it supports

there are some paid content on yandere, but they are deleted occasionally, you may need to turn off "hidden posts" option to view the explict content 

https://yandex.com/images 

It's good to find resouces can be used in templates, use filter to limit search results

https://github.com/lltcggie/waifu2x-caffe/releases

You can use this tool to scale the images if you can' find a large version, but the output will be slightly deformed

https://blacked.neocities.org/

You can use it to generate blacked membership card, if you know how to edit its source code it will be customizable

https://nijicollage.xyz
A Japan website incudes many templates can be used on editing, not all the resources are related to blacked theme