Image Classification with two very similar and random-ish classes
This is likely a combination of 1, 2, and 3. Without any data to back this claim up, 800 data points seems woefully small for the scale of problem you are trying to solve.

Assuming the problem is solvable, why not try to build a model directly onto the time series of the underlying data? What extra information are you getting from the image that you can't get from the time series? I would recommend reading up on some time series models.

Keep in mind that people using way more sophisticated methods are likely trying to build similar models, and likely are not succeeding.