The images and metadata still can be viewed on Opensea. But I know that Opensea has its own database which has stored the image and metadata already. My problem is if I created a maketplace by myself then after, can I see the image and metadata of my NFT on my marketplace? Is Pinata reliable for storing those data?
In case your marketplace directly reads the images from IPFS every time you want to show the nft then no, you would not see it if it is not in IPFS anymore. You could do what opnsea does, read them once and store them somewhere else, that makes your site render faster.
Now for pinata, yes it if very reliable. The thing here is that the free plan is rate limited, meaning you can only see x images in 1 minute and that x is low but it is good for development pourpuses. If you have a large project then I would recommend paying for pinata. If you don't want that you could also spin up your own IPFS node and pin your images.