The place where you are storing files is important if you have many differant files in differant file formats. Messiness can lead to disorganisation.
To make it simpler, formal and more organised there should be a folder for the main image that you are creating. Within that folder, should be two differant sections; Drafts and Final. The files in the folders should be clearly named i.e. image_ORIGINAL_450.psd. "ORIGINAL" means that you shouldn't edit the original and should make the copy to lessen the risk of losing your work. "450" stands for the dpi (Dots per Inch) in the image. "Psd" stands for the type of the file format.
Optimisation is used to get the best quality out of your image for the purpose it is needed e.g. using an image file thats 1GB for a large A2 poster, then reducing the size of the file for different purposes. Images on the web would have to be saved in a lossless file format like a PNG file. The resolution would also have to change to about 72 dpi.
Depending on the purpose needed of the image, you could change:
The size of the image
Quality of the image
Dpi of the image
File type of the image
Number of colours in the image