You can always estimate, if the image has reasonable colour depth (represents each colour value with high precision) you can take the conservative estimate of one bit per pixel (it is probably more than that) any image bigger than 16x16 probably has more than enough entropy.
This is what a totally random image looks like:

As an example, here is a very non-random image (most of it is blank, broad areas of colour, visible text, recognizable qr codes) that contains enough randomness to be a seed:

It is unfortunately not uncommon for people to use things like "Satoshi Nakamoto", "correct horse battery staple" and even "" as private seeds.
There is a well known case of someone putting 250BTC using "how much wood could a woodchuck chuck if a woodchuck could chuck wood" as a seed, and being hacked by a DEFCON hacker who later gave back the coins.
Someone else stole coins from the Bloomberg show, because someone was given a paper wallet by the host, and, after opening it, held up the private key to the camera for everyone to see it over live TV.

The lesson:
Private keys are called private keys for a reason, and hackers are willing to go through a lot of work to get them.