If you are travelling the web to find and download pictures you for sure will be successful. But the differences between how well the site works differ very much. There are sites around that will be a charm for you and you will love them for sure - and others are a pain in the ass!
After having discovered a site you find interesting, you should try to recognize what sort of comfort it has. Does it have ZIP-files? To answer this you of course can check the preview. But my own recipe for this is to check out review sites like Adult Reviews - a review has to answer whether a site has ZIP-files, how well it is suited for downloading or whether it is hampering downloads as much as possible. If ZIP-files are not mentioned in reviews, there will most likely be none.
My recommendation is plain and simple: Prefer sites with ZIP-files! If you want that comfort to become the standard, contribute to make sites using them successful and sites not using them a loser!
If you have subscribed to a site, treat it fair. If you like it - others will too, so don't spoil it for everyone! Do not share the login with others (you will lose it), do not redistribute pictures and try to use safe and long enough passwords that are different for every site you subscribe to, to protect yourself against password thefts.
If you have decided to use a spider you should be aware that this can be a both practical and dangerous tool that is not to be abused. You can minimize your risk if you handle it with care as described above. But if there are ZIP-files on a site, NEVER try to use a spider! And if the spider won't work - what should you do? Well download content from the site manually but consider not keeping your subscription alive for a longer period and not returning to this site before they move forward and provide the beloved ZIP-files!
And if a site uses DRM - boycott it! This will be the best and perhaps only measurement to prevent many sites from using DRM in the future.
Use your common sense - and have fun!
- Uli