I want to use a technique like text-indent:-9999em or negative margin to replace my text with more pretty pictures, but I wonder if search engines would treat this as kinds of cheating?
I think for it to know this, it must read my CSS file, so I can use robots.txt to forbid it from reading my CSS file. If this works, can I also use display:none directly? Or I can manipulate some web-element’s style by using jQuery code?
The techniques you have outlined (negative margins/indents) etc are common practices.
As long as you’re not introducing spammy links/text using these methods then you’ll be OK – it’s all about intent. Google has stated (via mouthpiece Matt Cutts in a webmaster video) that you should try to avoid this approach if possible and use traditional alternate text for inline images etc.
There are a number of different text/image replacement techniques floating around, each with different UI/UX/SEO impacts that you’ll need to consider. I’d recommend this resource as a starting point (bonus – there’s a link to a ‘Google impact’ post): http://mezzoblue.com/tests/revised-image-replacement/