how to prepare image dataset for training model?
as you want use pklot dataset for training your machine and test with real data, the best approach is to make both datasets similar and homological, they must be normalized , fixed sized , gray-scaled and  parameterized shapes. then you can use Scale-invariant feature transform (SIFT) for image feature extraction as basic method.the exact definition often depends on the problem or the type of application. Since features are used as the starting point and main primitives for subsequent algorithms, the overall algorithm will often only be as good as its feature detector. you can use these types of image features based on your problem:

Corners / interest points
Edges
Blobs / regions of interest points
Ridges
...
I have a project that use Deep CNN to classify parking lot. My idea is to classify every space whether there is a car or not. and my question is, how do i prepare my image dataset to train my model ?
i have downloaded PKLot dataset for training included negative and positive image.
should i turn all my data training image to grayscale ? should i rezise all my training image to one fix size? (but if i resize my training image to one fixed size, i have landscape and portrait image). Thanks :)