thanks a lot to all, without your help i would not have figured that out. you are great indeed. thanks once again

The Support article itself:
http://en.support.wordpress.com/images/#upload-images-from-your-computer

When you click "Add Media" in the post editor and upload an image from your computer or select an image from your media library, the right column includes an alignment option, a link option, and a size option. That's where you select the size you prefer for the version that will be inserted into the post. You select full size, large, medium or thumbnail.

Full size means original size, if the original is narrower than the maximum the theme allows (= the width of the main column), or scaled down to that maximum, if the original is wider. The other three options are supposed to be three different options for further scaling down. As I said, you set the defaults for these options in Settings > Media. You set the default width for thumbnails to whatever you prefer (150 is ok), and you set the default widths for medium and large to what's reasonable on the theme you're using. The width of the main column in Publish is 525px, so you should set medium width to around 250 and large width to around 400.

So, after you've done that, if you want to insert an image that will cover about half of the main column width you'll select the medium option, if you want to insert an image that will cover almost but not quite the whole available width you'll select the large option, etc.

about #3
thanks for your advice, i went to media-->settings and i saw thumbnail, med, large and full size settings.

but my question is, when i insert a picture, i can not decide what is the size of the picture??!! i do not have that option? i have an option to "edit" and then to rescale the picture or to crop it to re-size but then
how can I know if it is thumbnail, med or large size?

my picture size is 260 × 254 but when i inserted wp gave width =150 and the hight = 146 (see code below please).

The thumbnail is the media-->settings is 150x150,
the medium is 300 x 300

<img src="http://lezawang.files.wordpress.com/2012/12/jellyfish-e1355454686275.jpg?w=150" alt="Jellyfish" width="150" height="146" class="alignleft size-thumbnail wp-image-115" />

thanks a lot once again

2) When you edit an uploaded image, you create a new version but the original remains in WP's servers, because the changes you make are undoable: if you go to your media library and try to edit an already edited image, you'll see that the options include "Restore Original Image".

3) When you insert an image, you decide the dimensions, by selecting the desired size option: thumbnail, medium, large or full size. The defaults for the first three options are again set by you, in Settings > Media.

the edited one is here http://lezawang.files.wordpress.com/2012/12/lighthouse-e1355369328104.jpg

the original one is in my post and here is the url
http://lezawang.files.wordpress.com/2012/12/lighthouse.jpg?w=150&h=112

my second question, when i edit the post i see the picture width and high as following
width="150" height="112"

who decide these dimensions? these has nothing to do with picture dimensions when i edit it?

Thanks a lot.

It looks like you are using an outdated version of Firefox. Can you try using the latest version of Firefox (or using Chrome)? (The latest version of Firefox is 16.1 - you can get it at browsehappy.com.)

margin-bottom: 20px;

Save the changes in that window and then continue on with the other images and when done update the post. You can adjust the bottom margin value as needed. 20px seems good though.

Here is an example:
http://winkypedia.net/2010/12/30/thousand-year-old-century-egg-%e7%9a%ae%e8%9b%8b-a-chinese-delicacy/

There are odd times I manage to edit this by pressing 'Enter' over 100 times but this method does not always work neither. How can I solve this problem?

Thansk for your help.